Hmmm, it's tough to pick a single favourite level from all three games... but I could probably give my favourite level from each game, and perhaps then I could work out which of those three I like most.
I like
Orang-utan Gang too! Mainly for its fast-paced play style, challenging timing
(jumping over all of those barrels...), and that highly inventive bonus area hiding place beneath the start. Having a steel keg to ride is always neat, and then to get Expresso later on; very cool. The vines are fun to cartwheel along, and they add to the jungly look very nicely. The sunset looks great, too. Overall, it's a great level to explore, lots of fun to play, has a variety of game play styles involved, and some of the most interesting terrain in the game.
Then for DKC2 I'd have a much harder time, as I loved so many of the levels. I think
Clapper's Cavern would be my choice for its interesting gameplay concept, the fact that I adore Clapper, and the supreme challenge of completing the level without Clapper's help! I also really like the variety and flow of play in this level.
As for DKC3, still a fairly tough call, but I think I can happily select
Demolition Drainpipe as my favourite. It's just plain fast and fun!
It's still really hard to pick an ultimate favourite, but since DKC2 is my first love, I guess
Clapper's Cavern takes gold.