It has to be Hauptstrecke Rhein-Ruhr for me. Vast potential untapped there, especially with all of the new stock we have had added since it's release, namely ICEs. The game would have you think it's a rural branch line based on service frequency, which it quite obviously isn't. It's a personal favourite as any type of run - S-Bahn, InterCity or RE/RB - are long enough to be fun but not too long as to bore me out. More variety and AI services would go a long way to 'completing' this route.
I have got it to a point where it appears in the game, but only some services seem to be drivable (the ones that start at startup), anything starting in a portal doesn't allow you to spawn into it, and just spawns you at a random station. Not all "Portal" trains start at level load, but others do, so don't know whats going on, and any on the map don't seem to start at level load, and are still there, so block platforms and paths up. its up on TSC, if anyone wants to have a look and offer any feedback. I suspect its simply down to ticking the right boxes when creating the timetable, but don't know what combination is needed.
Its now working, and submitted for approval on TSC, so have a look and see what you think. Now the passenger side seems to be working, I'm going to start on the freight side (although first service shows up as empty rather than loaded wagons at present)