Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Sion. With his passive and Enrage it seems like he would be a very viable tank. However when you look through the guides, there is not one if the top 5 that builds straight tank. That is what this guide is for.
The runes that have been chosen are for pure armor and resist stacking. They provide flat increases so that early game, when you need the farm, you can resist skills to stay in the lane longer.
Tanks require defense. Hence an almost pure defensive mastery build with some utility to aid that.
Start of with a rush to Warmog's by getting Regrowth. This is so, if on the off chance you get hit early game, you can just sit behind your creeps and regen what you lost. Then on to Merc's cause as a tank you want to be able to free yourself of stuns and snares and the like so you can get back to tanking like a BAWS.
Start with Gaze. Early game a 2 second stun is slightly OP. Next is Death's Caress so you can take a bit more damage. Then Enrage and Gaze. Level 6 obvious ultimate is obvious. Then bounce back and forth between Enrage and Gaze when you can. That is to say, if you can put to levels, one after another into Gaze, then do so. Max out Caress last.
You want this build order so you can stun someone, neutralising an enemy, allowing you to soak more damage from the other enemies, therefore allowing you to tank for longer. Enrage helps by giving you more HP. Caress give minute shielding, but more HP and a 2 second stun are more important.
In summary, Sion has been designed by Riot as an obvious tank, yet people don't. It's time the community did. With a good spread of Magic resistance and armor, he can wade in, tank like a boss, and then walk out after the fight.
Now get out there and tell your enemies to GET ON YOUR CHOWPA