Best: I hate to do this, but I have two nominations: SMW and SML2. SMW was the original as far as Ghost Levels go. They were puzzling, creepy, but still somehow fun. SML2 may be weird for a nominee, but it was more or less the same as SMW, as far as the puzzling feature went. Both games had great level designs, which were (and still are) lots of fun to play through.
Worst: Mario Sunshine. I just turned this ghost into a block when I sprayed him. Now I can reach the next floor and- what?! Too low?! Mario 64 was a bit boring, but at least Mario Galaxy was a step back in the right direction.. the levels are nearly as much fun to play through as they were in SMW.
I nominate best first level.