Sion i'ts one of the best tanks in toplane in the meta because:
-Much Damage
-Good CC
-Very tanky
-Engager
The problems that have this champ:
-Poor mobility
-Weak Early
-His abilities can be dodgeables easily

Grasp of the Undying: Allows you to do good trades in early,, more maximun life per use and in late it's good because his damage is based on you're maximum life(4%).
Demolish: Gives you the chance of destroy turrets very quickly, because the amount of life you have.
Bone Plating: All the trades or 1vs1 with these runes are to your favor, because reduce the damage of 3 abilites or attack incoming.
Overgrowthing: Every 8 minions gives you 3 maximum health, after 120 minions a 3,5% of maximum health. This works perfect with the titanic hydra and demolish.
Biscuit delivery it's perfect for the bad early of sion, gives you a ton of sustain so just you don't have to recall so quickly.
Time warp tonic: Saves you with the half-potion heal a lot of times, and the additional movement speed too.

Passive: Don't kill minions when you are with you're passive because the enemy can freeze you and force teleport. Just use it to destroys towers if they are low healt and in teamfights focus the enemy that it's more closer.
Q: You can use it for push, pickups, and trades, you have to learn his hitbox because it's very false in a good way. Not always u have to charge it all, sometimes it's better click the Q and do damage in melee range or charge it a little bit.
W: Try to farm better you can, because the passive gives you maximum healt. The shield most of the times you have to use it aggressivel when the trade it's finishing. In teamfights it's so big that you have to activate at the start of it.
E: This gives you the setup for the Q, and reduce the armor of the objetive. Can use it aggressively with the Q or for slow when you are escaping.
R: You can use it to go from Top to Drake and being win-con in early tf. Also u start the combo with the R+Q+W.