Bob-omb Battlefield
Whomp's Fortress
Tick Tock Clock
Bower in the Dark World
This is an interesting one because I think Bob-omb Battlefield is really the only one that definitely deserves to be there. And as far as the Bowser levels go, I actually think Bowser in the Sky is the least memorable one. I think Dark World and Fire Sea are much more iconic, but I went with Dark World.
Ah, yes, the Negotiator:
General Kenobi
<sneaky beeping>