Sion 60% cdr ult is broken.
Don't get me wrong. I LOVE SION. I love that he is strong and I love Tilterella's lethal build. With the correct runes are cannot loss lane. Comet Ult Hat 10% cdr at 10min Burn Boots 5% extra cdr Many games have been won by 2 Sion ults in one fight. you have the ability to engage with it and 28 seconds later use is to end the fight. These runes are good on Sion no matter how you build him. If you're tank Sion max E and poke in lane. If AP still max E but get mana flow. If AD get 2-3 points in E for lane then max Q. Also I will add that if you want to go full lethal mode Dark Harvest might be the way to go, but still with ult hat and the burn. Doran's RING is the best starting item for the extra damage on his E plus the mana you get from it when not running mana flow keeps you in lane.
Here's my normal tank build.

Here's the best AP build that I have tried since the AP item changes.
(boots depends on comp really)
This is my favorite AD Sion build. Works mid, top, or even support (but you build ap support item and not Yomu)
(last item is situational) BT for sustain is death is amazing and will unexpectedly kill enemies. Last item Frozen mallet, deadman's, warmogs, iceborn, or randuin's is good too. Really anything.