From my perspective: Building Real Drives is most fun when i can use a lot of different trains. That means both freight, regional and Intercity.
So Nebenbahnen and Schnellfahrstrecken where there are only IC trains, are equally boring to me. In regards to schnellfahrstrecken with LZB betrieb. I play tsc to drive trains. Not have the trains drive themselves while i push the sifa button every 30 seconds.
Routes with traffic like Salzburg Rosenheim, Köln Koblenz etc etc are the ones i like.