Not Updated For Current Season
My first guide, I've played Sion a few times with this build and have never seen anyone else use it. Sion can be a very effective tank with some decent dps to boot, yet all I ever see is AP. With good use of runes and Enrage, you can easily get Sion above 4000 HP.
For the most part, you wanna go for the HP. I have a little bit of potency in there because the Sion's ult is lifesteal based off of damage output, so we want some power behind our blows in order to stay alive when everyone's beating you down, but that could just as easily be replaced by fortitude if you want.
For masteries, I go for the defense tree with points in the other two trees for ghost and exhaust. Nothing too complicated here.
Sion's tank item build tends to depend on the enemy team.
I first start off with Wriggle's Lanturn because the lifesteal allows me to keep enrage on indefinitely, keeping my max hp growing, as well as a free ward. This is an awesome item early on because you can keep going with enrage as well as see what your enemies are doing so you can prevent ganks.
For boots, I personally go with Mercury Treads because a lot of the time there is going to be some CC, and a lot of the time it will be focused on you, because that's your job as the tank. You could get away with Berserker's Greaves, but 9 times out of 10 you're gonna wanna not be stunned more than you want the extra attack speed.
A great item, it buffs a lot of necessary stats, such as your AP, AD and AS, allowing you to steal more health with your ult and keeping you alive for longer in a team fight.
This item depends a lot on the people playing. I chose to show Thornmail because carries tend to be melee, and they do a lot of damage, and they like to focus on Sion. Thus, Thornmail whittles down their health down to nothing before they realize it. However, sometimes their team is much more ranged and spellcasty. If that's the case, go Banshee's first, then Guardian Angel for the armor and magic resist, as well as the free revive.
This is a GREAT item. It increases your health and mana, both necissary for this build, AND MR. TBH, I almost find myself buying this before Thornmail, so it's very situational.
Late game, you are facing some pretty high damage output, so the extra health is always nice. Also, the snare effect is great for chasing down enemies.