Twisting Nether 2, short & sweet http://www.freeriderhd.com/t/1056-twisting-nether-2 And we can't forget City Scape: http://www.freeriderhd.com/t/1111-cityscape
FutureRootsLv3.2 is definitely one of my favorites. I like the level aspect. Also, the checkpoints are in perfect spots. Its a long one tho. Also, Parachute Drop is a classic!
Taiga is by far my favorite. The combination of a fun track and the art work is amazing! The amount of time and work that goes into creating these tracks is astounding.
http://www.freeriderhd.com/t/1267-lonely-thicket This track truly inspired me when I first started trying out detail. South was my motivation for detailing. I really miss him, I remember I won a collab from his contest on TrackMill like 2 years ago. That never happened lol. Looking back now that track isn't so great, it's not up to the standards some people have anymore, but it's still my favorite track. (Along with Monkie Park of course, TFM knows what's up.)
Catch ya plane is definitely one of my favourites. My brother and I would take turns and see who could get farther on it when we were younger. Good times.