I'm currently playing through my game (not done yet) so the movesets need work, as well as the fact that I don't recall the items they are holding at this point. Anyway, here is my in-game team.
Umbreon (Name: Deimos)
Everyone I know has used the Espeon. Being a more unconventional player I decided to toss this to the wind and use Umbreon. With access to tons of useful status moves and solid defensive stats it's an exceptional support Pokemon, although it isn't too strong.
Meganium (Name: Biscuit)
-Razor Leaf (Solarbeam)
Meganium is one of my favorite starters. Even though Quilava and Croconaw are better in most people's opinion, I have always favored defense over offense. It can stay on the field for a long time thanks to Synthesis, which is one thing it has over Umbreon, and Razor Leaf is useful against both opponents in battle.
Ampharos (Name: Blackout)
Ampharos is moderately bulky and has plenty of special attack to utilize. Thunder Wave and Cotton Spore are used to aid in catching Shadow Pokemon. Aside from Raikou it's the best Electric type you can obtain in my opinion, despite its low speed.
Mantine (Name: Lester)
Early game, Mantine is a fantastic Water type with loads of power and bulk behind it. However, as you go through the game, it's usefulness declines as other water types with far better movepools outclass it, especially those with access to Surf. I simply adore this Pokemon, so I've kept it all the way through. If you don't like my decision, look for a Water Type with Surf, like Quagsire or Qwilfish. However, if you do decide to use Mantine, it won't disappoint. Hopefully.
Stantler (Name: Randolph)
Are you tired of a far-too-heavy focus on bulk and not enough physical power? Yeah, so was I. I didn't really think this through. Anyway, those problems were remedied by the entrance of Stantler! Although it's a tad late game a physical force was very much welcome on my team. While Take Down has recoil problems it has a lot of power behind it and Hypnosis helps catch late-game Shadow Pokemon. Although normally Stantler isn't considered, it's very useful in Colosseum. And did I mention Intimidate? It's one of the best abilities in Colosseum.
Flygon (Name: Ribbon)
-Slash (I think)
Another thing that was dragging my team was the presence of Electric type foes, especially Magnemite and Magneton. Flygon became my saving grace in this aspect. It has incredible offensive prowess along with decent bulk, and has access to incredibly powerful moves. Although it's available sort of late in the game and Vibrava is difficult to raise, Flygon's payoff is more than worth it.
So there's my team. Feel free to take ideas or edit it to your liking!