Mine are in no paticular order: 
 
American:
Williams Bay and North Western--a route I'm making, still a WIP, but track's all laid and i love running it.  Still trying to master the AI traffic patterns and timetable/train schedule when i'm to lazy to build on it.
IndustRail2010--heavy switching route (my favorite pastime when running my trains/trainz is switching)
PO&N--same, plus decent long haul
Avery-Drexel TS2010--best mountain railroad ever
UMR2010--just absolute awesome all around--don't try it with AI though

 at least not until you're good at it
Timber Ridge Line--best Narrow gauge route out there currently...I don't care what 
you think.  
 
British:
the Titfield Thunderbolt--based on the movie of the same name, it's another of my WIPs.  
The Lickey Incline--you have no idea how much work it took to make that thing work in TS2010 without S&C, but it was so worth it.  
Rosworth Vale--oldie but a goodie. 
And if i could ever get my hands on one of Casper's (Mezzoprezzo's) routes, that'd be on here too!