Discussions of this type can go on forever, but I'll throw in a few names/tracks:
mike204's 3-Way Paper Run
HotShoe's The Sandbox
Malibu350's EVO 1&2 to MTM 2 conversions
Phin's Alphaville
Winterkill's Wake Up!
Spreker's Button Hook
Hoop Dreams by Spazmonstr
Badger's Mother Lode 1.5
Fila's SLO Mountains
DavDemBlues' Extreme Luge
Rocketalces' LaPaWoP & LaPaMa MC/EF
Preacher's Preacher's Mines
...and a ton of others that I can't seem to recall at the moment. Mind you, the tracks you see above (as well as the ones that aren't named that definitely fit into this topic of discussion) are creative masterpieces on several different levels. They're not just visual masterpieces with specialized gimmicks. I've been playing this game for 4 years, and I'm still amazed at what these guys and girls are capable of doing with this game in terms of creativity and technological advances (if you will). With that said, I for one tip my hat to every track makers out there. You all are awesome!
At any rate, that's Copey's 2¢ worth.