Introduction
Hey guys!
I have been playing Sion for a while, both ap and ad, and I have finally decided what is the most op. In my oppinion this is how a Sion should be built and played.
With this build in 3v3 you should be able to destroy the opponents team without much trouble.
Skill explanation
(All ability information were found here http://leagueoflegends.wikia.com/wiki/Sion )
This skill is basicly Sions only real ''tool''. With this build its damage is nearly useless, however Sion would be a useless champion without the stunning effect of it.
Sion's gaze terrifies a single enemy, dealing magic damage and stunning it for 1.5 seconds.
Cost: 100 mana
Range: 550
Projectile speed: 1600
Cooldown: 12 / 11 / 10 / 9 / 8 seconds
Magic Damage: 70 / 125 / 180 / 240 / 300 (+0.9 per ability power)
This is basicly the ability that is there to save you, and to boost your tanking... If I had a nickel for everytime this ability saved me from ignite... I'd have about 'tree-fidy' :D
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.
Cast again after 4 seconds to manually detonate.
Cooldown: 8 seconds after shield expires
Explosion area: 550
Cost: 70 / 80 / 90 / 100 / 110 mana
Shield Strength/Magic Damage: 100 / 150 / 200 / 250 / 300 (+0.9 per ability power)
This ability is this Sion builds bread and butter. The AD from this ability is amazing and makes Sion almost unstoppable combined with his ultimate. And not just that, but it also gives him an increase in his max hp for each kill (minions, heroes, monsters etc.)
(Passive): Sion permanently increases his maximum health whenever he kills a unit with either his attacks or abilities.
Health Gained: 1 / 1.5 / 2 / 2.5 / 3 per kill
(Toggle): While active, each of Sion's autoattacks will deal extra physical damage at the cost of some health.
Cost: 6 / 8 / 10 / 12 / 14 health per attack
Extra Physical Damage: 25 / 35 / 45 / 55 / 65
This ultimate is so freaking amazing! It gives great boost in attack speed and a mad lifesteal! In third level this ability gives 100% lifesteal! But its also great for your teammates because that it also heals Sions allies around him wih up to 50% of the damage dealt!
(Active): Grants Sion lifesteal and 50% attack speed for 20 seconds. Additionally, Sion's autoattacks will heal surrounding allies for a percentage of the damage dealt.
Cost: 100 mana
Cooldown: 90 seconds
Area of effect: 200
Life Steal: 50 / 75 / 100%
Heal: 25 / 37.5 / 50%
Pros / Cons
Pros:
High damager
A great burst damage
Extremely hard to kill
Cons:
Very vulnerable to exhaust and ignite. Especially late game
Can be easily harrased late game
Item dependent
Huge mana consumption
Farming
With Sion its extremely important to get a great farm. I would go so far and say its more important with Sion to get a good farm than it is with a hero like Nasus. You can often differ a good Sion and a bad Sion on their cs. About 25 min in, whether its 5v5 or 3v3, you should have about 100 cs.
This is also why I chose to have all flat ad runes. Simply because its easier to last hit with a high ad early game.
Alternat items
The only items in this build that you can trade out is the boots, the thornmail and the guardian angel. For this I ofcourse have a few suggestions.
Is there an amazingly good playing Veigar or LeBlanc that instantly kills you when you enter the fight? No problem! With this item you should be able to withstand the burst damage and get up in their faces and start that lifesteal!
With this item you have a potential of infinite ad, because that with every creep you kill you get 3 hp and the more hp you have the more ad you get. Besides of that, with this item you should get dangerously clost to 100% critical chance. A part from that it also gives a little bit of armor and why say no to that?
This item is a bit of a gamble. Even though you are able to get a lifesteal up to 120% with it plus a passive 20% life steal and some bonus ad then a good AD carry will just be able to shoot you down before you even get close to them. I only recommend this item if the enemy team doesnt have any good AD carry...
Team Work
Sion is ofcourse most suitable for the solo lanes where he will be able to farm alot. But when it comes to the teamfights, its Sions job to initiate with a well placed stun for SOMEONE ELSE to initiate. Remember a Sion should never initiate with other than that stun, otherwise he will be focused and killed before he can deal any amazing damage.
Therefor when you are sitting in the lobby and about to pick Sion, always make sure there is someone else on the team that are able to initiate I.e. Amumu or Singed.
Apart from that always remember that its your job to keep the AD carry busy with you!
Early game
This is the period in the game where Sion will be quite useless. I recommend you to play safe and only last hit until you get at least level 5. The farming in this period is not that important when it comes to hp farming. However it is very important when it comes to getting those two zeals. You see with this particular build Sion first starts to get good after the first of the zeals, and after the second he shows why he is op!
Mid game
This is the period of the game where you are supposed to hp farm like hell! This is where the rest of your game with Sions is decided. If you get pushed alot on your lane, make sure to have ganks there and if that isnt possible then trade lanes or go jungle. If you arent able to farm here, then you will do more good afk than in the team fights.
Late game
If you get this far in the game, then congratulations! You are indestructible! If you have chosen you follow this build with every item then the ad carrys will have a really hard time to get you and the AP carrys will be of your greatest concern.
Also look out for on-hit champions such as Teemo or a on-hit Kayle. If they have build themselves with items such as wits end, malady and Madreds bloodrazer then they will be the ones you should fear. Especially a good Teemo... God I hate those mushrooms -.-'
Summary
The main things you should remember are:
1. The farm mid game is much much more important than a few kills on the other lanes. Always remember its not your job to gank the lanes but the junglers
2. Always be aware of ganks. If you dominate your lane, remember not to gank too much and maybe buy some wards
3. Always keep the AD carry busy with you in teamfights. This little hint can mean the difference between win and loose for your team.
4. Get that blue buff! One of Sions biggest problems is his mana consumption. So once in a while when you have pushed too much and you have to get back buy anyway, then make a quick visit at that blue buff.
This was my first build ever on this site :) please come with as much constructive criticism that you might have (and please note that english isnt my first language so please dont pay too much attention to that)
Edit 04/24/2012:
Made a little change in the runes and the skill sequence!
Build Guide
Discussion (3)
I choose this item over Dorans shield simply because that little life steal there is in it makes the life cost of 
The double zeal start is what makes Sion deadly! This is his mid game steoroid that makes the enemy rage quit. The strenght of this item isnt in its crit chance, but in its attack speed and movement speed. Because what makes Sion a very weak champion is his ability to chase enemys. The only thin he has for this, is his stun. But in mid game with the boots his movement speed should lie around 450 which is more than enough for chasing and landing those stuns.
This item is so amazingly good for Sion. I cant even describe how much it actually helps him! It just makes him generaly better, plus it makes his burst damage amazing... But what really means something here is that it also slows the enemys he attacks.
This item is what makes him ''The critical threat''. With this and the trinity force his crit chance will lie around 40% and about 230 AD. Not to mention the huge attack speed and movement speed it gives him.
Well... What is there to say? It just makes him able to kill a caitlyn in four hits... No big deal...
This is his end game items. In most cases you wont even get this far in the game. But if you do, then this is the item I recommend. Basicly when you get this you are already abou 35 minutes into the game and ofcouse you have remembered to farm alot so 
If you get this far into the game you will most likely be first or second priority focus when it comes to team fights. Which is where this item comes in. It doesnt just make you that little bit more difficult to kill and makes you revive, it also has a threatning effect of seeing that full build Sion with 300 ad and 100 % life steal and 2 in attacks speed with that white ring around him.
Our very own huehuehue champion. Go I hate this champion so much when I play Sion -.-' ... He is simply too good at harrasing. However it all depends on the skill of the Mordekaiser. But basicly Sion will have no chance against a Mordekaiser that plays defensive.
I just hate this champion so much! That blind and those mushroom completely destroys Sion! This build makes his movement speed great... NOPE! Teemo put a mushroom! Sion has 100% lifesteal and 300 AD... NOPE! Blind. ... And when you finally get up to him, a good Teemo will blind you, run the hell away, make you run into a mushroom and then make you rage quit and cry...
This champion will be difficult to be against due to his ability to harras you alot. Up to level 6 you wont be able to do much against him. However after level 6 you will be albe to push him back.
What do I have to say? Huge armor and with a thornmail and 3 sec taunt is just not funny...
His ability to harras you is just simply too great. Being against this guy on a solo lane can prove quite tricky.
His only way to attack you is to get all up in your face. And if he gets up your face you will destroy him wih a huge ad and a shield that counters his alpha strike. If he is the solo lane then for the love of god counter him with Sion!
... All you have to do is to dodge that damn axe and not to let him farm... Problem solved.
After level 6 this guy wont be much of a problem. Just play defensively against him and you will be good.
This guy can be quite tricky, but if you use that sexy 
Just like Master Yi she has to get all up in your face to deal you damage. Which will prove fatal for her. The only real problem with her is her 
ability.
