I love discussions of this kind because I love talking about tracks. At the same time, to limit myself to just three is impossible; there are soooooooooo many awesome tracks out there!
At any rate, allow me to throw a few names/authors at ya that haven't been mentioned:
Alpine's Crazy - Alpine
Wake Up! - Winterkill
Extreme Luge - DavDemBlues
SLO_Worlds - SLO_HotShoe/SLO_Fila
Castellar Mountains - ObeOne
Jumping Screwballs - Finne
The Grade - OLD_Hotshoe
Lakebed Extreme - Valvejob (this one's not for sissys lol)
Preacher's Mines - SLO_Preacher
Superbowll II - ONaN
Y2K Supercross - NKAc_duuks
Moparz Mayhem 1 - SRT_Moparz
Valley of Kings - Guitar Bill (another tough one, but it's simply awesome)
Crosswinds - Psyco (simple layout, but the racing is phenomenal)
Spazmania - Spazmonstr
Paper Run - mike204
Mind you, this is just a small sampling of my favorites, but see what ya think. Some of them you'll hate, some you'll love. If you're still playing this game in 3 years, you'll find that some of the tracks you used to hate are actually incredibly good lol. At least that's how it was for me.
Lastly, some of the tracks listed in this thread have replays made by SLO_Jumper and a few others to help you figure out the best way to approch learning the race line of a particular track. Check out Team SLO's
Hall of Fame