Peach's castle, Saffron City. Peach's castle just offers so much room for creativity with the bumper and the floating triangles, the upper platform being permeable on both sides (the only one of its kind that big), and strategy involving the moving bottom platform. It's almost impossible to camp here because the level is so dynamic (even the bumper and triangles are moving); the best position to be always changes. Sometimes you'll want to be on the left side, sometimes the right. Among the people I play are two very campy players so I love this level. As for Saffron city, it's a classic.
Then, Dreamland
Then Hyrule (would be higher if it wasn't overplayed), Kongo Jungle (would be higher if it wasn't hard to see sometimes)
I used to have Sector Z up there (as I imagine most people), but over time its coolness just wore down...and ended up being too big. Plus the Arwings sometimes come out of nowhere and do a z2d lol
Then Zebes...I've come to appreciate it over time.
Mushroom Kingdom is another one of those "liked in the past, but not anymore". I have never liked Yoshi's Island both in the past and now. Never will. Everything just seems stretched a bit TOO far. The platforms are a little too far apart from each other, the main platform is a little too angled, the left blastline is a little too close, the clouds are too far apart, the amount of time they last is just the right amount to be awkward...it's like if one day you entered your room and everything was just slightly angled strangely and it's no big deal at the moment but over time it just drives you nuts.