My favourite routes:
Denmark 2000 electric - very good detail and almost perfect because it's realistic and covers several possibilities (eg. Korsør + Nyborg old stations)
Lollandsbanen II - fantastic detail, a real 'gem'. There's a lot I haven't looked at yet....
NDK - my first Danish route
. It is a shame it's not under development any more, but it has been responsible for a lot of good Danish MSTS work!
SDK - somewhat replaced by DK2000 but still has good signalling system.
Other routes I have installed at the moment:
LGVMed 3.0 - just to try out the TGVDuplex. The route is very dull compared to DK2000 - very little detail
Wupper Express 9 - A good route but I haven't enjoyed it much yet
Other routes I have used in the past:
MidEastUK - 1977 London King's Cross - Retford (East Coast Mainline) - very good route but getting old now. The author (Tim Court) is working on Train Master TrainSim
Inlandsbanan - not very detailed!
North West England - fairly good but I haven't used it much
Denmark 1b - good for S-trains and Malmø