I've wanted XD for the longest time... but I was stuck with Pokemon Colosseum D:
However, I really enjoyed Colosseum, it was the first Pokemon game that introduced 3-D graphics, and I always enjoyed the battles. I remember playing it the first few times and not being able to beat the last few people in the game, as my team was always too weak -.-
But then I learned about competitive Pokemon and oh dear.... I still can't count the number of times I reset for a good nature on a certain Pokemon, and good IVs, oh god... so much of my life wasted lol...
But seriously, Colosseum was probably one of the best of the Pokemon spin-offs.