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The stock team most people go with for the beginning, is
Espeon (is fast and hits hard)
Hariyama (takes hits and has Fake Out)
Typhlosion (rounded stats, uses fire moves)
Quagsire (SUUUUUUUUUUURF and Earthquake)
Ampharos (Electric is good coverage)
Misdreavus (for the first half of the game, the AI will be too stupid to understand what a ghost type is)
During the playthrough, you upgrade Typhlosion with Entei, Ampharos with Raikou, Quagsire with Suicune, Hariyama with Hitmontop and Misdreavus with something... better? (Flygon is good, because it learns Earthquake).
Other than that, all starters except for meganium are good (don't bother with grasstypes in this game, it's not worth it. They all suck and learn bad moves), Medicham can replace Hariyama AND Espeon, Altaria is okay (but needs help to learn good moves), Forretress is a great tank and pretty much all the endgame stuff (Heracross, Metagross, Tyrantitar etc.) is excellent, but, well the main game is over once you have them.
Remember that Pokemon Colosseum is 100% Double Battles, so once you reach the Under, go into the shop and buy Protect TMs for every single one of your pokemon (Protect + Earthquake is the best and it also lets you exploit opponents going for really obvious super effective moves). Grinding Mt.Battle for an Ice Beam TM for your Water type is generally a good idea as well, same for Flamethrower if you don't like Fireblast PP/Accuracy.