Summoners spells are pretty simple. Flash and teleport are your best options as for a reliable escape method (flash is the go-to for almost any champion) and tp for laning and transportation. Ignite is viable if there enemy wont benefit too much off of free farm while your gone or if you want to aggro sion. barrier is unecessary but certainly an option if you want double sheilds. same goes for exhaust, and heal if you have trouble with him or are just getting into. Do not take cleanse. Your ult is a cleanse. smite is for jungle sion ( always believe you can ult in the jungle, anythings possible!

Pros-

insane farm
great burst
can usually one for one
near global ult

cons-

unreliable stun and ult
passive easily escaped
people cant play him in ranked

Sion is a great farmer due to high aoe burst damage. Basically charge your axe, set up your sheild, slam down axe, blow up sheild to finish most of them, roar of the slayer to pick up stragglers.

sion is versatile and can max different abilities first, situation pending. Q is great for chunking damage and is best maxed out first on less mobile top laners who cannot escape it. TI like maxing e second for the armor reduction and ranged poke, however might come first ffor hard to pin down champs like riven. W is usually third for massive tankiness, but if you must play very defensively, it can be maxed first.

never stick to a strict one build. find what items you like and are good for the situation. is your team struggling and needs a tank? build tanky. can you afford massive damage to help carry the game and take objectives? build fighter. I encourage this time of playstyle very much so as any good player should do it :)

most of sions hard matchups are those who are extremely mobile like riven and can avoid is abilities or those who have powerful early game poke like g-plank. those who are weak early and mid are good as your burst is always strong at all stages of the game. so basically nasus, gragas, e.g, shouldnt be too bad. one that i found very odd was aatrox as hks early game is weak and you can aggro the hell out of him but his sustain and autoattack power is scary late game if he gets enough gold to buy items. DENY HIM FARM.