Sion is one of the strongest champions in game due to the fact that he can push out his lane then go roam with boots of mobility. With his shield scaling with a .9 ap ratio you can dominate your lane with it. Also he has an intimidation factor since he gets health with his Enrage.

I like to start with flat ap runes so you can get the early intimidation. With these you don't have to wait to long to start the snowballing. Helps you be able to clear creep waves faster.

With the new masteries utility is no longer as good. It is now better to go offensive if you are mid however if you are top I recommend going more defensive with masteries. The three magic resist is better than three armor because if you are mid you are most likely going to face an ap mid. With these runes and masteries you will start with 30 ap at level one.

You should start with boots and three health potions so that you can stay in lane longer and push more. You will already have enough ap with runes and masteries. When you go back you should get dorans rings. You should also get wards they are EXTREMELY important. A mejais is good for when you are dominating lane and roaming. Sion is naturally good because he can feed himself. Rabadons is good because it gives you a lot ap. You should get a Deathfire Grasp because it helps with your full burst.

Sion is strong early and late game so play him and wreck with him. Don't disappoint with this build.