This is my first time that i make a build/guide and i want it to make with a champion that i know im good with.
Sion is a very strong champion either way AD or AP, I start using this champion as AP and i like it because it was super easy to kill others by just using Cryptic Gaze and Death's Caress.

By using only AP runes and Magic Penetration, it gives you a start of 54 AP what at early game makes you almost invincible, and this AD runes are very good because you have everything u need for a good early game sion, you start with lot of attack damage and one of the most important one critical chance, than you have the attack speed runes that helps you a lot when you use your Cannibalism on early game.

I like to start with Zeal when i use AD sion because in early game is very useful, why? because, you start with good attack speed plus it gives you good movement speed. Than when you get Berserker's Greaves you get even more attack speed plus a very good movement speed.
For AP i love Doran's Ring at the beginning because thats what gives you the sufficient AP that you need to own them. After Doran's Ring you get Sorcerer's Shoes you get Magic Penetration plus movement speed.

For AP i like to use first Death's Caress so when you attack them you have already the cooldown to explode it, than you use Cryptic Gaze and there it goes at least half of their HP, and if you need some help use Cannibalism.
For AD make sure you ALWAYS have activated the Enrage than you do the same thing when using AP sion, the only thing always try to have your ultimate up before going to a battle.

Just in case you are fighting against AD champions like Tryndamere Master Yi or Gangplank you may want to get Thornmail as your last item,and if you are against AP champions like LeBlanc Brand Morgana or Malzahar you may want to get Guardian Angel as your last item.