Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Manamune - I know what most of you are thinking, "Why start with a mana crystal?" The answer should be pretty obvious; because it gives you mana. Sion starts with barely enough to let off two stuns before he's out. With Tear of the Goddess as the first buy and finishing Manamune as the 5th, there should be adequate time to fix that problem, and give you some bonus attack to boot.
Berserker's Greaves - The only other thing I could see being here is Boots of Lucidity, but the way I play this build, the extra second of cd off the stun/shield isn't a gamebreaker, and the extra attacks you'll get while cannibalism is up is more than worth it, especially early on.
Trinity Force - "Really?" Yes. Nothing in this item goes to waste for Sion. The slow down especially helps teammates keep up with a gank.
Nashar's Tooth - Loads of attacks speed, 20% cdr and some mana regen and AP to boot. Is there really any question?
Phantom Dancer - One thing I learned during non-ranked matches is that they do not expect Sion's fat *** to catch up to them. Extra attack speed and crit chance to boot = lots of ganks and a MUCH more effective ult.
6th spot is reserved for anything you may need to stay alive. If anything else, a Guardian Angle wouldn't go amiss; then you can backdoor with no qualms. Besides that, Banshee's Thornmail, Atma's, Guinsoo's; a lot of things can go here.
Sion starts with much lower health than he used to, so the runes I think are deserving of a good start for the game. The reason I came up with this build is because I found both AP and AD sion lacking in something.
AP - Shield goes down before you detonate, you die. CC works WAY against you. Cannibalism is near useless.
AD - Shield goes down way too quick, it becomes a waste of mana unless you are really good at divebombing. Cannibalism has a nice heal, but your stun doesn't do any damage; your shield doesn't absorb any damage. Most auto-attacker strategies can apply to beating you. Oh yeah, and where did your mana just go?
I feel this build is a good dps build because you can chase down quite easily and stun runaways so the team can catch up. Shield and stun can be used much more frequently and do more to keep you alive and your enemies dead. Cannibalism does its job well, and you can backdoor like a boss if need be, or cleanup other lanes of those pesky opponents.