Hmm, I guess Rico Harbour. Riding the bloopers (NOT squids >_>) and defeating the the giant one (man, I forgot his name) was fun. I also liked walking on those rafter like things. I pretend it's James Bond, I walk forward, a blooper confronts me, I take a precise shot and send it flying off the platform.
I also like finding Bowser, going through the lava and clouds. Fighting him was better, though.
It's been a while, hasn't it?