DKC 2 was the best one.
DKC 1's bonus stages were a lot more trial and error than anything. DKC 2 gave subtle hints so if you had a good eye, you could find most secrets. ('cept for that one secret in Galleon's Galley, ARGH) David Sirlin describes it better than I can: http://www.sirlin.net/archive/hiding-secrets-in-platform-games/
In DKC1, Donkey Kong was useless. In DKC2, Diddy's roll jump gave him a speed advantage Dixie couldn't afford. On the flip side, Dixie's glide was excellent.
DKC2's boss fights were a lot better. The final boss (the regular and the secret one) tested platforming skill. DKC1's tested...when to jump.
DKC2's Lost World was pure awesome. And it was first!
After DKC2, Rare was infected with collectivitis and forced you to go on a billion scavenger hunts. Drove me nuts.
There is no such thing as a console war. This is the first step to game design.
