So I recently bought Sion, and took a good look at his skills. From afar, it seems that with his 1:1 AP ratio, AP would be the definite way to go. But then I looked at his Ultimate, and saw that it would be best with an AD build. So, what's the best build for Sion, AP, AD, or even Hybrid? I was thinking AP would be best for 3v3, whereas AD would be best for 5v5, but I really have no idea, so some help would be welcome. Thank you.

Oh, and if it matters on my genera playstyle, then I can tell you that my most played character is rammus, so if that has anything to do with what I should choose, then please tell me.

I play crit/AS myself, but Ive seen many successful AP Sion's.

I haven't played Sion, but from looking at his skills I would suggest getting a few items that have both AP, and AD, but I would certainly go more for AD. I don't think you will have the mana to rely so heavily on skills with an AP build and I certainly would not build into more mana. Also since his only 2 abilities that use AP are a shield and signle target, AP won't be very effective in large ammounts unless you can spam his abilities, which again would require lots of mana. Short summary, AD over AP, but best to get some AP for shield.

Hextech Gunblade.

From what I've read and what I've seen, AP is the way to go for earlygame dominance. If the game moves beyond 30 mins, you might want to consider transitioning into an AD/Hybrid build as your ability to blow people up falls off quickly once people start getting survival items and are more focused on staying together.

The most annoying part about AD sion is his incredibly slow attack speed early game, usually you can do well if you build for defense early game and transition into AS (Your enrage is basically a free BF sword and then some) and lifesteal late game.

Sion's great because he really can go both AP or AD.
Personally I go AP because his burst is incredible.
Late game he becomes a major stunbot and his shield adds a bunch of survivability and harass. His damage won't do as much as early though.

TL;DR
AD is great late game and after lots of farm. (I like comparing it somewhat like a Trynd)
AP is pretty nice all around, drops off just slightly late.

Hmm, I see. Are there any good guides for AP or AD Sion?

Here is a good AP Sion Guide.

I'm pretty much repeating what's already been said but the difference between them is playstyle really. AP Sion is a mid-game best and AD Sion can be a late-game beast. I find AD Sion is alot easier to shut out completely if the enemy team is packing a single Exhaust, or saves a stun for you to prevent you using your ultimate for its full duration. AP Sion is alot harder to shut down as his farming ability is simply insane, allowing him to have a tonne of AP very quickly.