Not Updated For Current Season
Sion is a champion that has been around for a while now and he has been getting mixed values from Tanking, AP carry and AD carry. Out of the three thier worth to me is like this.
AD > AP > Tank.
For tanking purposes you have many better options such as Rammus, Malphite and the recently buffed Amumu.
AP carry is a good valid build which is great early game and doesn't really falther later on.
AD carry is a bit shaky at start but as soon as he gets his ult it really starts to pick up and doesn't really go down. His major weakness is that he has to hit to do things. CC such as stuns, blinds and to a lesser content Slows really hamper Sion's abilities. If he's allowed to rampage and hit stuff at will he can most likely take on enemies 3 vs 1.
-High movement and a heavy hitter.
-Great farmer and ganker.
-Anything without additional armor will go down within seconds.
-Needs to hit things to stay alive himself.
-Weak vs CC.
First off summoner skills.
I picked Ghost and Exhaust. Both are ideal for getting more melee hits off on enemies. The only other 2 skills I could see in this build are Ignite and Flash. Ignite would be great to get an opponent running back to ther tower to deal those last few damage and Flash for gettting suddenly into stun range. Overall I find that flash only gets you 1 or 2 extra hits at most where Ghost will allow you to chase someone long enough to get 4-5 hits off. This build doesn't drop an enemy in 1 large burst but rather in many repetitive smaller hits.
For skills I max Enrage first. Sion's biggest problem is his attack speed which is rather low at the start. By getting Berserker boots and Stark's fervor early (and then a phantom dancer to boot) I solve that problem and increasing his melee damage with enrage makes it so that his attacks actually deal damage. It also provides Extra hp which makes him a bitt more buff without needing to invest into HP items.
Cryptic gaze is a second choice to stun enemies in place to get more hits off. I often only use it to gank players or to intiate in team fights. AD sion does not have the mana pool or regen to spam!
Shield is rarely used. I use it as an "Oh sh*t!" button and later on to get that little buffer at the start of a team fight. Because you stack no AP you will not deal much damage with it. Most of the time it'll break before the 3 seconds are over.
Cannibalism is the main skill of course. You put points in it as soon as you can and you use it often. It's a great way to kill enemies because without disables you will most likely heal faster then you can be damaged. If you get harrassed hard during early game laning you can proc it at 6 and kill a minion wave to heal up and maybe surprise an enemy with it (+ stun and exhaust) and get a kill off.
I start off with a vampiric scepter. At level 1 Enrage gives you enough damage to get good hits off on minions but it'll hurt your hp pool a lot. By grabbing this baby you'll heal slightly more then you are damaged. You'll build it into a Stark's Fervor later on.
I quickly build up Berserker boots, Stark's Fervor and Phantom Dancer to get high attack speed. When you get those you can cut thorugh almost anything with your ult as long as you have your enrage on (put it on at level 1 and never shut it off!).
Frozen Mallet and MAdred's Blood razor are a or/or item. I pick the mallet when there is nothing that can resist my blows and Madred's if I face a tank with high HP/Armor/Both.
If there are no tank enemies you might as well build an Infinity's edge after the Bloodthirster. Or you could build another bloodthirster if you'dd like for more raw damage.
Try to last it minions for the enrage procs and money. Don't be greedy this is your weakest time. Many people will expect AP sions who are amazing early on but you aren't.
Unless they have a verry squishy with low HP who you can stun and gank you shouldn't try to harrass enemies. Be especially aware of ranged enemies.
At level 6 you get your ult and you should become more aggresive. Surprise your opponents in your lane by suddenly proccing your ult and smacking them back to their mommies. Don't be too aggresive though you still aren't killing everythign in sight just yet. After your phantom dancer you can start attacking ppl who are 2v1 but look out for those diables.
If your opponents aren't surrendering they must have some amazing CC to keep you at bay. You rip through things so fast it's not funny anymore (well it's funny for us). Lone wolfs should for no problem at all so if you can find someone all alone go for it and rip him to shreds.
Be prepared to get focussed down in team fights. You are best for going in the front but if you have a beefy tank let him go in first (yes even as carry) if you get shut down you are still capable of dieing. If they don't focuss you enjoy getting tripple kills all around.