Before many people played tank Sion, but now he is neglected because of the advantages of playing AP ( high burst dmg, and op shield ), or AD ( sustained dps, ulti that heals whole team ). These are great, if you like dps, but because of Sions passive he makes an effective tank, and a fun one as well.

-I first take Cryptic gaze because this is what Sion is all about, the stun is great and the dmg is nice. When the item build is finished you will have extra cd reduction which will help these ability to stun multiple enemies in a single team fight.

-Then i take Enrage to help me with farming, and because, with this build, you can keep this ability on all the time and lose little hp.

-Sions ulti Cannibalism is one of the strongest abilities you have even with a tank build bacause you can keep attacking and healing your allies and yourself. Use this skill when your low on hp or when initiating in a team fight.

-I take the shield in the end, because you have a lot of hp, mg ress, and armor and whitout AP your shield is not worth much.

I start Sions build with Dorans shield which gives you great staying power, then when i go b i buy a Rubys crystal and upgrade it to Levathian. Levathian pays of because you rarely die with tank Sion, and you get a lot of assists which will fill your stacks up nicely. Then i go for Randuins omen for the cd red and nice ability that can help your team greatly. Aegis is the next item that will help your teams squishys greatly. The last item is Sunfire cape that gives you extra hp and dps. If the game drags on you should sell Dorans shield and get Force of nature for the hp regeneration, movement speed and epic magic ressistance.