What makes Galio such a fantastic champion to play?
Galio is next to the best Anti-Mage tank in the game. The only other comparable Anti-Mage tank I have ever seen is Alistar based on his high use of MR and healing. Both champions excel in the way they work. Together they are an unstoppable force.

Who is the Anti-Galio then?
Well this all depends on play style, generally any Attack Damage based champions can hurt Galio pretty badly, but in most cases if he is being focused: A - Either his teammates are nowhere nearby; or B - The Enemy Team is stupid and will be taunted on to him and nuked down by his team.

- Galio is, in his own way, the best Anti-Mage tank in the game.
- Galio excels in starting openers in team fights and making them turn in his teams' favor.
- He can push lanes very easily if harassing the enemy champions with his Q->E Combo.
- Excellent Creep Farmer.

- Not the best tank to use against Attack Damage based carry champions.
- Slightly squishy early game, if not careful.
- Requires you to know how to use "Skill Shot" skills. (If you are bad at these then don't use Galio, because he is based 70% off of Skill Shots)
- Can lose Mana very easily.

Well the reason I choose Flat Magic Resistance and Mana Regeneration / 5 (At Level 18) is because he get Ability Power from Magic Resistance and he has a very hard time with mana sometimes. Now if you are good at knowing how to harass and knowing when you can, then you can substitute your Mana Regeneration / 5 with any of the following: Magic Resistance, Magic Penetration, Armor, Health, Cooldown Reduction, or Ability Power.

The reason I choose to go 6/3/21 is because with this build I rarely run out of mana, I have slightly more Ability Power, and my Cooldowns are a lot shorter than the other Galio Builds. So I rarely have to recall, or wait for my spells and kite people. Another nice thing with this build is the bonus Magic Penetration from Offense Tree. This makes up for not having Magic Penetration Runes.

Honestly, I choose to use Clarity and Ghost because I view them the best use in my play style. Ghost is amazing for either catching an enemy champion, ganking, or even escapes. I grab Clarity because then I can spam my skills a lot more often and not feel limited by the low mana pool Galio has. Although you can choose whatever you are most comfortable with.

I chose this line-up based on a typical match that you would run into 90% of the time. Doran's Ring is great for the Extra 100 Health, 15 Ability Power and 5 Mana per 5. Rushing Mercury Treads is extremely effective as by level 6 it will help you with making sure you can finish off opponents. Once you have your treads I would rush Catalyst the Protector as quickly as possible so that you can start using skills even more often. As you finish this build you will notice you have generously high Armor, Magic Resistance, Mana Regeneration / 5, and Ability Power levels.

Honestly it all depends on the other team's builds. If you are going against Tanks and Mages, just grab a little more Ability Power through items such as Rod of Ages or Archangel's Staff, I list both because they can either give you more Health or more Ability Power/Mana. Just choose which one you feel is required. If per say the team is heavily Attack Damage Built you can grab Aegis of the Legion, Frozen Heart, Thornmail, Zhonya's Hourglass, Mercury's Treads, and Guardian's Angel. This build will give you high Armor, mediocre Ability Power, and some decent Cooldown Reduction. As it is said, "Just go with the flow."

After reviewing what I posted before I can agree that it was very "vague". So I am going to do my best to give as much detail as possible.
Galio is not a FIRST BLOOD champion, do not under any circumstances go into bushes to try and get first blood with your lane partner.
Situationals!
A: Lane Partner is a Carry - If this is true, let him/her get 4-5/6 last hits on minions and you get the other 1-2/6 last hits. This is very important, yes the carry should get built first, but you need the gold as well, otherwise you can not fulfill your role(s). Do this until your carry has roughly 20-30 minion kills then split the last hits down the middle as cleanly as possible. As before, during this point of the fight you and the carry should try to harass the enemy lane champions as much as possible, you would primarily harass with your Resolute Smite ability.
B: Lane Partner is an "Armor-Based" Tank - If this is true, I recommend starting off immediately at a "3-3" Minion Kill split. That said, you should still try your hardest to harass as much as possible with your Resolute Smite Ability. If your Armor-Based Tank is Malphite you'll be in a great spot, have him harass with Seismic Shard a lot and you'll be able to push back the enemy very easily. Here's why: Seismic Shard at level 1 does an immediate 70 Damage, if your Malphite has Armor/AP Runes (Generally a lot do now), he will do about 78 Damage a Shard. Add in your Resolute Smite's damage: 80 Minimal at level 1. Or with my build it does an immediate: 101 Damage. So altogether that's 150-179 Damage everytime you both use your Shard/Smite abilities. 5 sets and this will kill any carry that isn't being healed enough.
C: Lane Partner in a Support Champion - I honestly hate support champions with Galio and don't recommend it, but if you have no choice, then all I would say is have your support just keep you healed while you harass the enemy like crazy, nothing more to say because I just flat out don't recommend this comp.
D: Lastly, solo Top/Bot (Jungler) - If you have no choice but to do this I would recommend you play very defensive, use your Resolute Smite to last hit multiple minions and use your Righteous Gust to Harass Enemy Champions. Honestly Galio isn't very good solo, so try your hardest not to be on the 1v2 lane.
Anyways, onwards!
How to build your items in Early Game Phasing: Basically it's the same as before, get Doran's Ring and rush your Mercury Treads, the sooner you can get these babies the sooner you will start to do a **** load of damage. If you have to recall and don't have the money to fully buy your Mercs just get some Mana/HP Potions. (1 or 2 of each) Also buy a Ward. The ward would be used primarily for your lane, but if the mid laner seems to keep having people try to gank him/her, I would put a ward by them to help them out. Remember, your objective is to be completed as a team, think about everyone, not just yourself!
Now we will talk about enemy compositions: There are 7 compositions for this.
A: Support/Support - This setup is annoying as hell, pretty much just run them over. Harass them, wreck the minions, and push the turret, their damage is nowhere near as much as your guys', so just play very aggressive without being stupid. Knock off about 400-500 damage per turret whack. Once done, kill them, then go on and do whatever needs to be done next, nothing more needs to be said.
B: Support/Tank - Another rather annoying setup, I recommend harassing the Support in this setup because the tank will just absorb most of your damage. Stick with the minion splits mentioned above, based on your comp, and play O/D (Offense/Defense) strategy. Only push if you have the upper hand, otherwise stay back and last hit minions.
C: Support/Carry - This is a typical setup and most people have a good idea of how to take on this setup. My take on it is, harass the living hell out of the carry, kill minions, and harass the carry some more. Try to play moderately aggressive, but be defensive as needed.
D: Tank/Carry - This is probably on the most difficult setups to counter because they can do quite a lot damage very quickly while taking less than most setups. I recommend harassing the carry when can, but play moderately defensive in this setup, just don't let them hit your tower if you can help it. Otherwise, it's just a game of push and pull.
E: Tank/Tank - These guys will be a literal push-over, they look like they're hard to kill, but if you focus them one by one they can't do anything after a small while. Treat these guys like a double support setup, be strictly aggressive.
F: Carry/Carry - This setup is the counter comp for almost every comp you can find. You want to be very careful against these guys, just play a defensive harassment game with these guys until you are level 6, then wait til one of them is in a really bad spot, use your Righteous Gust ability and go ult him/her.
G: ****/Jungler - My recommendation for this is to cut off the solo champion from all XP as much as you can. Just get between him/her and the minions and keep him/her at the tower. Other than that, get a ward every now and then to keep an eye out for the jungler. If you see him/her coming, take him/her out and then retreat behind minions/to tower and heal up a little. The best thing you can do is catch a jungler off guard and kill him/her early game. This can determine the outcome of a game.

Mid Game is now considered as "One to Two Towers taken down". By this point you're usually at the 10-12 minute marker. Pretty much there are several things that are going to be happening by this point. You will start to see more team fights with 3-4 people and 1-2 people pushing a seperate lane, unfortunately you can't help stop those 1-2 because you will be a huge part of the team fights. Your objective is to taunt any and all carry/support champions with your ult and get them taken out immediately. What about tanks? Well, they're a waste of time unless they're way out of position. If that's the case, slaughter the bloody bugger(s). No tank means that you can easily push side lanes without any problems, just choose one and put 1 champion on each of the other 2 (preferably a tank/carry). Try to have at least 2-3 wards in their jungle as well. This helps keep you aware of where their people are going. If you see an opportunity to kill someone, DO IT.
Ok now for what items to start working on: This depends on the enemies, I recommend rushing your Rabadon's Deathcap and Catalyst the Protector. These items will help you a lot more than you might think, a single Deathcap with a Catalyst means you can do a ton of damage while being able to ultimate at anytime.
As a side note, I recommend you buy an Elixir of Brilliance every 5-6 minutes if you can afford it, this cooldown reduction will be very helpful in team fights, as you will be able to harass a LOT more. Also get a blue buff if you can. This makes your cooldown timers rediculously short, it becomes slightly "overpowered".
Anyways, last thing I can say about this time in the game is to try to get your Abyssal Scepter and a Negatron Cloak. Try to have these few items by the time there are a total of 6-9 towers down and roughly 20 team kills.

Ok now for Late Game strategies!
By this point you should hopefully have the following items:
Doran's Ring, Mercury's Treads, Rabadon's Deathcap, Abyssal Scepter, Catalyst the Protector and a Negatron Cloak. If you are missing any of these items, get them asap and then work on finishing your Banshee's Veil. At this point all you have left to do is get a Guardian's Angel and you are set for the game. Pretty much you will be the 2nd person to jump into a team fight and you will be the person who tries his/her hardest to ult as many of their squishies as possible. Don't ultimate if you can't get 2 of the 5 enemy champions, instead just harass with your Righteous Gust/Resolute Smite abilities, and use Bulwark on the lead person/main carry, whoever is lower on HP or further ahead.
WHAT TO NEVER DO AS GALIO LATE GAME: NEVER LEAVE THE TEAM FIGHTS! YOU ARE NOT TO BE GOING AROUND AND CREEP FARMING, THIS IS A NO-NO! YOU ARE A TANK, LEAVE THE DAMN CREEPS FOR YOUR CARRY(IES), THIS APPLIES FOR YOUR SUPPORT/TANKS ON YOUR TEAM AS WELL. The tanks/support champions can get minions in the main push lane while fighting the other team. I can not stress how much of a waste it is for a tank/support to be taking creeps from the carry. Unless they are 100% built on gear do not take creeps from them! Otherwise go ahead, but I still recommend staying with your team as your role is such a large key to winning team fights.

v1.0.0.122 - 2011-07-26
•General
◦Base health increased to 520 from 495
◦Base armor increased to 20.5 from 16
◦Armor per level reduced to 3.5 from 4
•Resolute Smite
◦Projectile speed increased to 1300 from 1200
◦Mana cost reduced to 60/65/70/75/80 from 70/75/80/85/90
◦Ability power reduced to .7 from .8
•Bulwark
◦Duration increased to 4 seconds from 3.5 seconds
◦Heal increased to 25/40/55/70/85 from 20/32/44/56/68
•Idol of Durand damage reduction while channeling increased to 50% from 30%

v1.0.0.120 - 2011-06-22
•Idol of Durand will now correctly increase its damage when the damage taken is shielded

v1.0.0.118b - 2011-05-24
•Idol Of Durand's damage range now matches the taunt range

v1.0.0.115 - 2011-04-11
•Resolute Smite
◦Damage increased to 80/135/190/245/300 from 60/115/170/225/280
◦Ability power ratio increased to 0.8 from 0.75
◦Cooldown reduced to 7 from 8
•Righteous Gust
◦Movespeed increased to 20/28/36/44/52 from 20/25/30/35/40
◦Righteous Gust now applies the movespeed buff to Galio more quickly
◦Damage reduced to 60/105/150/195/240 from 65/110/155/200/245

v1.0.0.114 - 2011-03-28
•Idol of Durand area of effect increased to 550 from 500

v1.0.0.113 - 2011-03-15
•Righteous Gust mana cost reduced to 70/75/80/85/90 from 80/85/90/95/100

v1.0.0.110 - 2011-02-01
•Resolute Smite's area of effect size and projectile speed have been increased slightly

v1.0.0.106 - 2010-12-01
•Resolute Smite ability power ratio increased to 0.75 from 0.7
•Righteous Gust ability power ratio reduced to 0.5 from 0.65
•Righteous Gust movement speed bonus increased to 20/25/30/35/40% from 20/24/28/32/36%
•Idol of Durand
◦Ability power ratio reduced to 0.6 from 0.8
◦Damage percent increase on hit reduced to 5% from 10%
◦Total damage cap reduced to 140% from 180%

v1.0.0.105 - 2010-11-15
•Idol of Durand
◦Fixed a bug where casting would break an enemy's Spell Shield and then taunt them again
◦Fixed a bug where the taunt particle sometimes played on targets that blocked the taunt
◦Fixed a bug where Idol of Durand's ability power ratio was inconsistent
◦Ability power ratio decreased to .8 from 0.85
•Bulwark duration reduced to 3.5 seconds from 4
•Righteous Gust base damage reduced to 65/110/155/200/245 from 70/115/160/205/250
•Righteous Gust ability power ratio reduced to 0.65 from 0.8
•Resolute Smite ability power ratio increased to 0.70 from 0.6

v1.0.0.103 - 2010-10-18
•Righteous Gust mana cost increased to 80/85/90/95/100 from 70/75/80/85/90
•Resolute Smite mana cost increased to 70/75/80/85/90 from 60/65/70/75/80
•Idol of Durand mana cost increased to 100/150/200 from 100/145/190
•Idol of Durand duration reduced to 2 from 2.5

v1.0.0.99 - 2010-08-24
•Idol of Durand
◦Taunt radius reduced to 500 from 520.
◦Damage radius increased to 520 from 500.
◦Adjusted team colored range circle to more accurately represent taunt range
•Resolute Smite's particle will now more properly represent the area of effect

Release Notes v1.0.0.110 on 2-1-2010
Galio

* Resolute Smite's area of effect size and projectile speed have been increased slightly

Release Notes v1.0.0.106 on 11-30-2010
Galio

* Resolute Smite ability power ratio increased to 0.75 from 0.7
* Righteous Gust ability power ratio reduced to 0.5 from 0.65
* Righteous Gust movement speed bonus increased to 20/25/30/35/40% from 20/24/28/32/36%
* Idol of Durand
o Ability power ratio reduced to 0.6 from 0.8
o Damage percent increase on hit reduced to 5% from 10%
o Total damage cap reduced to 140% from 180%

Release Notes v1.0.0.105 on 11-15-2010
Galio

* Idol of Durand
o Fixed a bug where casting would break an enemy's Spell Shield and then taunt them again
o Fixed a bug where the taunt particle sometimes played on targets that blocked the taunt
o Fixed a bug where Idol of Durand's ability power ratio was inconsistent
o Ability power ratio decreased to .8 from 0.85
* Bulwark duration reduced to 3.5 seconds from 4
* Righteous Gust base damage reduced to 65/110/155/200/245 from 70/115/160/205/250
* Righteous Gust ability power ratio reduced to 0.65 from 0.8
* Resolute Smite ability power ratio increased to 0.70 from 0.6

Release Notes v1.0.0.103 on 10-19-2010
Galio

* Righteous Gust mana cost increased to 80/85/90/95/100 from 70/75/80/85/90
* Resolute Smite mana cost increased to 70/75/80/85/90 from 60/65/70/75/80
* Idol of Durand mana cost increased to 100/150/200 from 100/145/190
* Idol of Durand duration reduced to 2 from 2.5

Release Notes v1.0.0.99 on 08-24-2010
Galio

* Idol of Durand
o Taunt radius reduced to 500 from 520.
o Damage radius increased to 520 from 500.
o Adjusted team colored range circle to more accurately represent taunt range
* Resolute Smite's particle will now more properly represent the area of effect

Release Notes v1.0.0.98 on 08-10-2010
Galio The Sentinel's Sorrow is born

* Resolute Smite: Galio fires a concussive blast from his eyes, slowing and dealing damage to enemies caught near the impact point.
* Bulwark: Galio shields an ally Champion, increasing their Armor and Magic Resistance, and restoring Galio's health each time that Champion suffers damage.
* Righteous Gust: Galio claps his wings, unleashing a gust of concussive wind that damages enemies and leaves a directional draft in its wake that increases ally movement speed.
* Idol of Durand: Galio assumes the form of a statue, taunting nearby enemies and storing concussive energy as they attack him. Galio then bursts from his statue shell, releasing the stored energy to damage surrounding foes.
* Runic Skin: Galio converts 50% of his total Magic Resistance into Ability Power.

http://www.youtube.com/watch?v=rRzIE67LZkM&feature=related (By Colbycheeze)
http://www.youtube.com/watch?v=R4KRnJhkh7w&feature=related (By Colbycheeze)
http://www.mobafire.com/league-of-legends/build/galio-insert-something-clever-here-12392 (By Ezekial)
http://www.leagueoflegends.com/board/showthread.php?t=485937
http://www.lol-patch.com/Galio.html

Thank you to Colbycheeze and Ezekial for your wonderful guides.
Thank you Riot Games for allowing the players to see your Release Notes so that we understand any and all changes to champions we enjoy playing.
Thanks to JashobeamZ for pointing out that my Lane Phases weren't very descriptive, hopefully they will be easier to learn from now.
I will update this as often as time permits.

As always any and all criticism is appreciated, but please try to show me why something should be changed, so that I will be able to include your comment in the Credits.

Thanks,
demoneka