I play Sion for a long time, i found the best way to play him is AP. Much AP. I havent seen a good ap build for sion at mobafire, so i'd try to introduce u my best one.

Pros

Great dmg
Useful at teamfights
Bruiser

Cons

Without good lasthiting/at least 2 kills at early it is difficult to get eq

Yeah, the god of farming. U need to lasthit well to get some good cash and pwn totally at mid/late. Try getting Death's Caress on u, and hit as many mobs lasthit as possible with Enrage, to get some additional hp.

First, to pwn at your lane, u need to have a good help from your team. Best way to get some free kill at early is a help from mid lane before the minions appear at here. With Sion u need to have a good comunication with your team, ping everything u do, try to tell ur buddy what to do before an attack. Good comunication with team affects with good k/d/a for u and ur lane friend.

Standard ap runes, except seals. I think that there is nothing to explain.

Once again nothing to explain, a caster masteries.

Yeah, thats something new. First of all, u need dorans ring for some early ap and mana regen, most useful at early so i always take it. When u only get 1100, go fast for your Sorcerer's Shoes for some more magic penetration, and ofc movement speed. Sheen... This item is an enterance for full pwning at your lane. After that u can go Archangels, or end Lich Bane, its your choose based on situation on the map. I recommend to take Archangels if u have enough money after taking sheen and getting b, but if u need no mana, take lich bane. When u complete those 2, I prefer to take Void Staff for more destructible force! If you are doing really well, u get Rabadon at 30-40min. At the end of itembuild, u shall have 1 more place (selling doran's ring). Everything is about the situation on the map - Haunting Guise shall be a good idea if u go on max m.penetration, or u can get some def like Abbysal Scepter or Zhonya Hourglass.

I think thats the most weird thing about this build, but i tried it so many times, and it works really fine. At early u need stun, and max Death's Caress as fast as possible. After that im getting 1 stun lvl, 1 enrage for some better hp gaining from farm.