This is a tough question, as there are so many excellent routes now available for MSTS.
It all depends what you prefer:- British - American - European. I like to dabble with all three.
My list of preferance would be.
British:- Freeware:- Dorset Coast, Thames Mersey, Cannock Chase & Scottish Central, the latter being for the fantastic Forth Bridge. Although this route was never finished with full scenery, it still ranks one of the best main line stations ever built for MSTS ( Edinburgh Waverley) I have added quite a bit of scenery myself to this route, to make it look better once going North of Rosyth & South of Waverley. & as a Tram Enthusiast as well, Blackpool Tramway. Manchester Metrolink. Crich Tramway Village.
Payware:- Cambrian 2, Severn Valley.
American:- Freeware:- Moffat Tunnel (Rollins Pass), South Park Narrow Gauge V4, Marias Pass GN Hi Line, Whitefish V7.
Payware:- 3DTrains Feather River (Brilliant) - Maple Tracks Kicking Horse Pass. (Canadian)
European:- Freeware:- Abula V2 (Swiss Route)
Payware:- Line 51 (Berlin 1930 Tramroute) This is a most unusual Route, as it runs quite separate from from the normal MSTS set up. The Route Files are installed direct into the main Train Simulator Folder & the Trams into the Trains Folder. Although you have to have MSTS installed to run this route, it is a completely stand alone game (if you can call it that).
You have to do each Activity in turn, then save the Evaluation, before you can continue to the next Activity. You start off as a trainee Tram driver, delivering equipment to various locations. Once you have completed the first five or six activities, you get more parts of the route unlocked. Until you have completed ALL ( 30 ) Activities, you wont be able to Explore the Route as completing the activities unlocks more sections of the whole route system. Once this is acheived, you can get into the AE to make up your own activities with a good choice of Trams & Works Vehicles to choose from. So you have to actually treat this as a Career Type Mode of running the route, otherwise you just cannot do anything with it.
Whoever, fathomed out how to make a route like this, it is very clever and there is even music which can be turned on or off when you are playing the activities. The scenery is brilliant with some pretty good sound effects too.
If you want something different and interesting, it is worth buying this route, even though it is a Tramway Route, it is well thought out and put together. Oh and you cannot get into the Route Editor or copy any of the files from the Shapes or Texture Folders, because all the objects don't have names, they are all numerically coded, to stop anyone copying their material. Give it a try. Now reduced in price from
http://www.train-world.com
Geoff