Pokemon Colosseum still is and always will be my favourite pokemon game.
I got it when it first came out (around the time I just got my gamecube) and I was so engrossed in the story as a kid.
It had a grown up and serious udnertone, criminal activity, mafia-big boss type of danger.
The little amount of pokemon made it personal to use and purify them all alongside your companion who can see shadow pokemon.
The protagonist was cool and mysterious, never seeing his backstory other than that he's probably a skilled orphan who surived on the streets.
And best of all, it gave the most beautiful and memorable track for me that I ever heard in my life.
When I first walked into the Relic Forest to purify my pokemon, I actually stopped playing and just stared and listened.
Even listening it to it now brings back such pleasant and warm nostalgic feeling almost to the point of tears.
I love this game.
https://www.youtube.com/watch?v=o-K-b5ODBpg