Welcome to my first guide I've ever made! I'm going to introduce you a healthy monster champion, Sion. He can be played as ADcarry, as well as AP, but don't forget he can be a monster of a tank as well. You will see why...

+ Enrage ability gives you PERMANENT extra health on any kill you make: minions, champions or jungle monsters, and this stacks INFINITE!
+ Enrage also gives a decent attack bonus output for free.
+ Almost impossible to kill by anything at high levels, as you will sometimes get even more than 6000 health while having some decent armor and magic resistance as well.
+ Can solo dragon at low levels, and tanks Baron greatly for your team.
+ Ultimate can heal himself, heal teammates, and drill the enemy to the ground. Cannibalism OP!

- EXTREMELY MANA HUNGRY! Only use the abilities when you really need to.
- His passive Feel No Pain has almost no use at higher levels, as it only negates very little damage.
- Has no great escape mechanisms other than being very tanky and his Q.
- Vulnerable to pokes.
- Weak to Ignite when activating Cannibalism.
- Rather useless passive
- Expensive build.

Yes, there are more Cons than Pros. But you can say that Enrage's health increase counts as 10x a Pro, because it's a great, great ability for a tank.

For Sion, I take these runes:
Greater Quintessence of Percent Health, because you can expect to get more than 5000 health easily during one game, making this a better rune to choose than Greater Quintessence of Health.
Greater Glyph of Scaling Magic Resist, because you're a tank.
Greater Seal of Armor, because you're a tank. I don't take Greater Seal of Scaling Armor, because you really need some tankiness at low levels to reduce the damage of pokes/basic attacks at low levels.
Greater Mark of Attack Speed is VERY essential on Sion, because he gets very little attack speed per level. No attack speed=less damage.

For Masteries, I choose 9/21/0, because it's a tank, and to deal some damage as well, plain as simple.
I spend 3 points on Perseverance, just to regenerate some more health back that you lose from Enrage, and to get better sustain in your lane.

I spend 3 points on Hardiness to get a good sustain on early levels and to protect you a little more from invasions. Resistance might be a good mastery to spend points on as well, but I consider it useless in late game, because it only gives you 5 Mres. Hardiness might be considered useless on later levels as well, but it does make some difference on low levels.

I spend 4 points on Durability , because you're a tank.

I spend 1 point on Veteran's Scars , because you're a tank.

I spend 2 points on Relentless to get away from ganks better and to make sure that I can chase better without getting slowed. Also helps you again to protect yourself better from low levels invasions etc.

I spend 3 points on Tenacious , pretty much the same reason for the points I spent on Relentless

I spend 3 points on Juggernaut , because you're a tank. AND IT'S SO GREAT ON SION!

I spend 1 point on Reinforced Armor , because you're a tank.

I spend 1 point on Honor Guard , because you're a tank.

Starting up with 2 Rejuvenation Beads, a sight ward and a Health Potion might be the best start for Sion. The Rejuvenation beads give a great amount of sustain early game, and it refills your Health lost on your Enrage ability. Also gives a nice sustain in your lane combined with the 3 points spent on Perseverance. The only place you should put your sight ward is seen on this map:

Put it NEAR the bushes, not inside of it. That helps you and your teammates to locate the jungler better, as it gives more vision over an area when not in a bush. You don't need to locate the jungler inside the bushes, because junglers love bushes and like to hug them, so you know where they are already... you only have to wait for him to come out.

Item building

The first item you're going to build is Frozen Mallet. Why? Because it has great health, some decent attack damage and a 100% chance on a slow each basic attack, making this a perfect item to combine with your ultimate. However, if you're not winning the lane, you should take Warmog's Armor first to tank and recover the damage.

Why Frozen Mallet?
If you hit a champion, you need the movement speed to keep hitting your enemy. But... that doesn't work if the enemy has about the same movement speed as you do, so he would get away very easily. Frozen Mallet is a requirement for Sion to win the whole lane.

Why Warmog's Armor?
Because you're a tank.

Which boots should I take and when?
After getting your first item, buy your boots. Upgrading them is pretty situational: if the AD carry is fed, buy Ninja Tabi. If enemy has hard CC, buy Mercury's Treads. Just examine the enemies a little bit and make your choice. If you don't know what to buy, just hold on your Boots and wait for it, or take Berserker's Greaves for your minion farm(or if you're already winning).

Why Atma's Impaler?
Well, because of this.
Play Video /league-of-legends/item/bloodthirster-60 Why Banshee's Veil?
It's great against spells that can either do a lot of damage like Javelin Toss or preventing your death by preventing a nasty chain of CC abilities. Also gives you some mana, which good for Sion as he is very mana hungry.

Why Guardian Angel?
If you have like 6000 HP, and you die, you will revive with having around 2000 health. That's a pretty decent amount, and combined with some nice magic resistance and armor there is a chance to escape. It just makes Sion even more annoying for the enemy. Also gives nice armor/mres bonuses!

Other items to consider/not to consider.

Frozen Heart is a nice item against fed AD that gives HUGE armor bonus, cooldown reduction and mana which is also a great item on Sion. Cooldown reduction is needed to spam his Death's Caress even more. Also grants the enemy an atk speed debuff, which can help the team out a lot. However, you get a risk to not be focussed at all, so I only take it as a situational item, despite being a great item on Sion.
Sunfire Cape is an item taken by many tanks, but I think Atma's Impaler might be a better item on Sion, because it gives the same armor bonus, but also gives a huge damage boost based on your maximum health, something that we are building constantly, making you useful in teamfights, because you can heal your team with your Cannibalism. Also gives a Critical chance, which is always nice to have.
Mercurial Scimitar is a nice item against an enemy team that has a LOT of CC stuff, because it has a free Cleanse spell with a shorter cooldown. Gives some nice attack damage and Magic Resist. Although, this item is very expensive, and I only take it when it is really needed, because you can put your money into better items.
Only if nobody else is building it, replace banshee's with runic bulwark.

At level 1, I unlock Cryptic Gaze first to escape invasions or to get more chance on first blood while invading, as it stuns the target for 1,5 seconds. That is long enough to walk away or to get the enemy where you want him to be: death.
At level 2, I unlock Enrage to start building my health bonus. Note that you can do some nice constant damage already at this level, so if your opponent is already at low health, you should try Cryptic Gaze + auto attacks with Enrage.
At level 3, I unlock Death's Caress to block some damage of the pokes. However, do not use this ability too often as it will drain your mana very quickly. Only use it when you already lost some health and you are good on mana. You are good on mana pre level 6 when you have 220 mana.

First I max out Enrage because of it's passive that gives him permanent health.
Then I max out Cryptic Gaze so you can spam this ability more often, because the cooldown is reduced by 1 second each time you level up this ability. At max, you can stun enemies for 1,5 seconds each 8 seconds, which is really OP. Also note that this is one of the best non-ultimate stuns in the game!

Here I am going to explain what Sion's abilities do and the use.

Passive: Feel No Pain: Sion has a 40% chance to ignore up to 30 (lvl 1) / 40 (lvl 8) / 50 (lvl 13) damage each time he is hit by an autoattack. The damage reduction is calculated before armor and percentage damage reduction benefits are taken into account.
You ignore damage. That's fine. But it is too little to call this a good passive, because if you have 50% reduced attack damage at level 13, you will only ignore 25 damage because it's calculated BEFORE any reducers come into place. Oh well, at least it's something.

Q: Cryptic Gaze: Sion's gaze terrifies a single enemy, dealing magic damage and stunning it for 1.5 seconds.
This is mainly used to get away from an enemy or to deal damage along with your toggled Enrage. Note that this skill costs LOTS of mana, so save some mana for the escape!

W: Death's Caress: Sion surrounds himself with a shield which absorbs damage for up to 10 seconds. After 4 seconds, if the shield has not been destroyed, the ability can be cast again to explode and deal magic damage to surrounding enemies. It will explode automatically after the 10 seconds have passed.
You can either use this as offensive as well as defensive. But because you're a tank, you are going to use it defensively, just to block some damage. Note that it will always deal the same damage, no matter how damaged the shield is, so detonate it when there is little left of the shield. Note that it uses a lot of mana, again.

E: Enrage: While toggled on, Sion has increased attack damage at the cost of some health for each autoattack. While Enrage is active, Sion permanently increases his maximum health whenever he kills a unit with either his attacks or abilities. This effect is doubled against champions and large units.
This is the skill what makes tanky Sion so unique, it gains permanent health bonus when you kill anything. Due the fact that it is infinite bonus, you only get more and more tanky the longer the game is. This may result in crazy health, possibly reaching over 7000 when you are full build. It also offers a nice attack bonus to farm minions better, so it's a wonderful ability on a tanky sion.

R: Cannibalism: For 20 seconds, Sion gains bonus lifesteal and 50% attack speed. Additionally, Sion's autoattacks will heal surrounding allies for a percentage of the damage dealt.
This is a great offensive Ultimate as well as a defensive/sustain ability. 100% lifesteal when at level 16? That's OP as hell.
When using it offensively: try to use it as a harass. Just walk to the enemy, get hit by some damage, then the enemy is like HA! you're dead. Then you activate this ability to say HA! nope. But make sure that the enemy has Ignite on cooldown, because it's a very cheap and annoying counter to this ability. I always use this in combination with Ghost and Frozen Mallet, to make sure the enemy can't get away (unless it's Master Yi...)
When using it defensively: activate this and farm minions to get health back. Note that you can also heal your teammates with this when they're standing really close. When you heal your teammates, watch out for champions with AoE damage, like Lucent Singularity.

You should focus a LOT on farming, because each time when you get something dead with Enrage active, you gain permanent health. Try to last-hit every minions you can hit to maximize the use of this ability. To achieve this, try to get your lane pushed to your tower and play very defensively.
To farm efficiently while tower hugging:
Melee minions: let the tower hit them twice, then you do the final strike.
Caster minions: let the tower hit them once, then you do the final strike.
If not tower hugging(need level 2 Enrage):
Melee minions:: hit them at 1/4 of HP.
Caster minions: hit them at 1/3 of HP.
With this, you should get to last hit many minions. I know that from later on the minions will become stronger, but this is pretty much a simple basic up to level 11-12 to farm in your lane.
Note that it is very important to farm as many minions as you can, because your Enrage is based on it! You can also use Enrage's passive to farm champions, if necessary.

As primary choice for spells, I take:
Ghost: Cannibalism has a very cheap escape: distance. I always use this spell to combine this with the ult. Also gives you another possibility to escape ganks.
Flash: Because it's flash.

Other choices to consider instead of ghost:
Exhaust: To tear down the ADcarry from the enemy. Also used to escape ganks etc.
Teleport: Quickly go to base and come back to get more permanent health. That's great!
Cleanse: Great for CC enemies. However, I recommend you should buy a Mercurial Scimitar instead of this because ir's basically the same.
Ignite: Could be a great choice as well, but I've seen that you can do way more damage when you take Cannibalism + Ghost.

Don'ts:
Clarity: Even though Sion is very mana hungry, you just don't need it, because you won't spam your abilities.
Barrier: He already has a free barrier. Beside that, you're a undying beast.
Heal: You have enough sustain in the lane, so it won't be necessary
Smite: Only if you want to be über-efficient in farming, but is rather useless in later game.
Clairvoyance: JUST NO!
Revive: Too long cooldown, useless. Don't take it. Ever.

When a teamfight is approaching, you should go in first. Don't be afraid to lose a little health, and you should tell the others to stay really close to sion, because you are going to activate your ulti which will heal your teammates! However, if the enemy has very strong AoE damage, it is not worth to stand close, because you will all suffer a lot of unnecessary damage.
Use Q on the enemy AP in the beginning, because most of then will do crazy damage in a small time with ability spamming. 1,5 seconds stun should be enough to kill him if the AP is squishy.
Use W whenever you can to block most damage.

In short terms: Cannibalism > Cryptic Gaze on AP > spam Death's Caress as much as possible.

Here I am going to tell which champions are no match for you and which champions you should fear. Keep in mind that you're vulnerable to pokes, so if you have a pokey champion on top lane, cry for the ganks.
Well, I've played once against him, and I was countering him like crazy. You can cancel his ulti by many things: your health, with a good timed Cryptic Gaze (and if you didn't time it well, you can use Flash to get out of range) and with your shield. Beside that, he has no escape route as well, he only has one slow to use, but you have a 1,5 seconds stun to get in range again. It should be very easy, even if you're less fed than him.
It's your counter. The only thing you can really do is build armor from the beginning and beg for ganks. He can constantly poke you with Dragon Strike while you only have your Death's Caress to block some of the damage. Also his passive Martial Cadence is just the ultimate counter for Sion, as it does damage based on %health. Play defensively as possible, cry for ganks, pray to god, w/e. Try to build Frozen Mallet and finish him off with Cannibalism. Make sure you have Flash always with you to escape his Cataclysm when necessary.
A good Teemo is a dead Teemo. Teemo can constantly poke you with a basic attack, so you might want to play very defensively. In early levels you are quite vulnerable against him, but notice that his Blinding Dart has the same amount of blinding time as your stun does. Once you have the Frozen Mallet, he isn't speedy enough anymore to get away, unless he's putting ability points in his Move Quick, but... no Teemo does that, really. In short term, Frozen Mallet=dead Teemo with Cannibalism activated.
-- in progress --

The most important things to focus on in a nutshell:
- Farm as many minions as you can.
- In the beginning, don't focus on the enemy champion at all, only when he gets very close.
- When you have Frozen Mallet, try to win the lane with Cannibalism + Ghost + Cryptic Gaze.
- Don't forget that this tank can deal quite some serious damage.
- Play defensively in the beginning, as you're quite vulnerable to pokes.
- When avoiding death, Cryptic Gaze, Flash, Frozen Mallet, Death's Caress and Ghost are your escapes to choice from. Make sure you have enough mana!

When in teamfights:
- Get others to stay close to you when they don't deal that much AoE damage. Your Cannibalism doesn't only heal yourself, but others as well, but within very small range.
- Use Cryptic Gaze on the fed person.

I hope you learned something of my guide, and I hope you are going to victory with this guide. Please leave the feedback on this guide, it could use some improvements here and there, I think.