I always thought CD was the best. Best atmosphere and mood, most adventurous use of the mechanics. Game tries to actively stop you from going fast, but you still can, you just have to know how. Feels like it's better to play after the main games (1->2->3k) so you really know how to use the mechanics. Both soundtracks rule, Tidal Tempest is killer in both. Also, best special stage, imo. Definitely most fun.
2 is definitely the weakest, baring maybe S&K? Too much filler. The beginning is amazing, end leaves a lot to be desired.