Man, it's been so long I hardly remember. I loved most of it, frankly.
Espa is the level I remember most, though, besides the first level (because I played the demo a trillion times). I also really liked the swamp creatures running about.
Least favorite had to be the anticlimactic final battle tied with Otoh Gunga Jumping Puzzle of Doom. (I used slowmo every time I got there.)