I don't play LoL for a very long time, but I think I'm experienced player who want to share his experiences to others. I think Sion is one of best champs to fight 1on1, but there isn't many players which are playing as Sion. Idk, why. Op ulti makes him invicible in 1on1, stun which always strikes target, shield which makes a lot of DMG and it's good for farming and can has so much hp cuz of his E skill. I decided to write short build for this champ.

I think most importants spells are flash/ghost depending on which map do you play. I often play 3on3 so I choose flash to teleport through the walls. On 5on5 ghost is better choice to leave some ganks or catch an enemy. Second is exhaust. Sion is ad which has stun on Q. 2 cc are very good option, cuz firstly stun, ehxaust then stun again is nearly won game in 1on1.

IMO most important runes are for Armor Penetration and crit chance. Sion gets a lot of attack speed from his ulti, but you still need some items for ad. Best shoice is armor penetration then.