When this game came out, a Pokemon RPG that was (i) on a console, (ii) was in 3D, and (iii) had a real story were revelations. Looking back, the game definitely had its flaws, especially the incredibly limited roster of Pokemon. Nevertheless, it's so fun to play that I forgive it most of its flaws, and it set a standard for what I wished we could get for more than a decade after.
Pokemon Colosseum is really interesting, it puts a spin on the mechanics of the mainline series while still managing to feel like a pokemon game. The double battles are a nice change of pace, and the heavier focus on characters and more difficult battles over exploration and collection is a good fit for a console entry in my opinion. However, the game is absolutely bogged down by how slow it can be. There's too much dialogue, and the animations for some moves, especially the multihit ones (looking at you surf) are egregiously long. Other than that, good game.
Kinda gets grindy at a certain point, but otherwise one of my favorite Pokemon games next to BW2. A super interesting and fun take on the series that forces you to have a fuckin weird ass team by the end. Good ass TUNES. Good ass FIGHTS. Good ass GAME. PLEASE give me another all double battles game, Game Freak, you keep making so many funny doubles moves that can only be used in VGC.