Sion may seem like a troll pick to many but he has such a powerful kit that he can deal massive dps and still be impossible to kill. With his Enrage E he stacks hp which is massive in this game as you can get 1,000 hp when you stacked for 30 minutes. Then with the Boots of Swiftness you move quite quickly that only gets better. I shall go more in-depth with it later on in the guide. So please read on!
Edit:each extra build has a mini guide on it telling what its about.

Get attack speed since you dont need ad all that much (you should have 100). also it allows you to 1v2 quicker. I love his passive early as you dont need to get flat armor runes because of it. Scaleing mr runes since you have alot of early mr.

I havent experimented all that much with masteries so if you find one that works well please tell me and i will try it. these are basically just what i use on all tanky brusers with crits.

Build Ravenous Hydraasap as this is your bread (the quicker you get it the easier and better chance you have to 1v2) then Boots of Swiftness and upgrade Alacrity whenever you have the money. Then build as much damage as you can get off without getting blown up. once you do get some resistance to it. or use a combo on one of the other guides. (top of the guide with all the sion heads.

You dont need any more points in Cryptic Gaze q then 1. You receive no benefits. Get Enrage e for more scaleing on health and for that sweet 25-65 ad. Get Death's Caress w next. I cant tell you how many times this has saved my life. It will block 300 damage and with your ultimate that means 600 more damage you tank. This also is great early with that 100 extra damage that will block ignites and sometimes even will give you just enough health to survive a level 5 or 7 fight. Your Cannibalism ultimate is pretty self explanitory 100% extra lifesteal. The only thing to talk about there is that your 50% attack speed is great for killing turrets when splitpushing and allows you to 2v1 level 6. the range on the 50% healing is very very low so when you have someone like or on your team just stand atop of them with phantom dancer and heal them.

If you fall behind then just get Ravenous Hydra Phantom Dancer or static shiv and strait tank. If even with all your items you still die dont be afraid to sell everything and go strait tank (no Ravenous Hydra). likewise when your fed build nothing but damage and facerape 3 or 4 people.

pros
+has great sustain and passive tankiness
+has tons of raw damage in his kit
+is able to deal with almost all his counters
+with only hydra is able to instantly farm a wave
+has a 20 second ultimate with a 90 second cooldown.
+is able to make up for an afk or even make a teamfight basically 6v5
cons
+you have more counters then a kitchen in master chef. (item wize)
+when you fall behind you will only be a supertank.
+you will get kited reguardless what you build.
+you have no real excape
+you are absolutely terrible in 60 minute games.

Dont be afraid of pushing a wave early. The junger will most likely gank you 3 minutes in and as long as you didnt let the cs wave hit the turret they will act as another champion. also your passive Feel No Pain comes into play here as every 2 or so autos will be blocked almost entirely. Also minion agro will be ignored.

Use Cannibalism your ult to heal your frontliners. Use your reistance to tank backliners. Use your Cryptic Gaze q to peel for your carry. spam Death's Caress w for free 600 hp.

Not only can sion fulfill every single role in team fights but he can also do them simultaneously. He can peel tank deal damage engage and even support. Also he has insane early game damage and farming skill. He has possibly the biggest and strongest power spike in the game at level 11 as he can 1v4 if you have 3 items at this point. At level 13 it wears off. You can also win 4v5s by being this mid game megadamage tank.

This is a champion that can do almost anything but you will have to be HAVE TO BE.good with him to use him properly. This is comeing from a platnum 5-3 (depends on lag at the time) sion main. http://www.lolking.net/summoner/na/35748339#profile this is my profile.
Now feel free to go and tell me if theres something i should try.

The summoner spells are pretty necessarily on sion. Exhaust lets you be able to deal with burst potential and allows you 2.5 seconds off cc that you need for chasing. Flash lets you get away with having low mobility. you can flash stun to blow up a carry. flash over walls to escape a bad splitpush. or flash over skillshots to stop cc. don't use it to escape fights. your Cannibalism is more potent then you think

reserved for anyone that gives me a idea to improve my guide. please list your champion main so i can say what type of playstyle you are.
+more champion counters
+added spell chapter