The most difficult track? It is the track that I have the least amount of laps on in the particular car class. Simple as that. Every track is unique and every track has it's particular "difficult" section.

If you do not have a WR time, then those tracks are the most difficult. If they were not difficult, then you would have a WR time.

The question really is, what is "difficult" being defined in relation with? On normal public servers, Weshill in the FZR is extremely easy. Get those few guys I see now and then that run 1:30's and 1:31's and it suddenly becomes a very difficult track as I need to step it up to keep up.

I don't agree with that argumentation.
I got within 1% of the WR of As Gp Reversed in the FZ5 within about 20 minutes, and still I consider it a lot harder than f.e. Westhill/FZ5, where I'm still not below the 1% mark after some 180 laps.
Difficult is easiest described as "How difficult is it to stay consistantly fast?"
On Westhill my laptime-deviation is only caused by fuel consumption. I consistantly get 0.05 to 0.1 quicker each lap with a deviation of 0.2 or fewer. On Aston Gp Rev there is often quite some deviation in my laptimes (measured relatively to the absolute laptime), because I didn't quite hit the ideal line over some blind crest.

That's why South City Town Reversed is so difficult. Lots of blind corners, braking over crest, bumpy turn in zones, it just takes a lot of concentration.

Vain

They are all challenging in their own way. Though I don't see any of them as overly diffacult.

We just raced WEI rev in the fo8 challenge series and I'd have to say that's the hardest we've done so far going a realistic race distance after a pretty reasonable selection of tracks. (Harder than SO Long or FE Black were last season because of having a hard time finding a good compromise of a set with too many high speed rights burning up the front left tire and going just beyond a reasonable tire temp running r2's no matter what I tried)

FE Black... Just when you start to think there is some flow to the track, there immediately is a sharp turn which takes you by surprise...

Kyoto oval in MRT is so difficult, getting all that power and torque down.