This could be a good subject if you separate it a little further. How about the hardest single seater/track combo and the hardest enclosed/tin top car/track combo?
Also, since the single seaters don't really belong on the rallycross courses, those combos shouldn't be considered.
The UF1 at KY1. Maybe that shouldn't be considered either.
Now for my selections? I haven't driven each car enough to give an honest answer. However, I've driven a few cars a lot. I'll come up with something when I get more time with each car on each track.
But I'm guessing for the single seaters, it's going to be the BF1 due to the shear acceleration it has and the speed that it can reach before entering a corner and not being able to actually feel it in the seat of your pants.
For the multi-seat cars, it's probably going to be the RAC due to it's washy feeling. I'm sure it can be set up better, but I haven't spent any time with it yet.
As for the tracks, any track where you can't see the turn coming up until you are right on top of it can be tough until you learn the track real well. I've found that to be a problem with netKar Pro, most of the turns you can't see until you get right there and then I'm off into the grass with an unrealistic sound of the car riding on the ground. Most of the LFS tracks seem to have good visibility of the actual track surface and there is enough scenery to gauge from, except for maybe some turns at KY2 and KY3.
-Maurice