I don't know, really. Most (no pun intended) of them (especially the street circuits and certain parts of other tracks, e.g. the uphill parts of Sonoma) have become more difficult since S3 started expanding the damage model.
Le Mans (or la Sarthe) is hard to get consistent and fast at the same time, due to the length and the sheer speed you get on the straights just before the slow corners (All the high speed straights ends in a slow corner or a chicane except for Porsche and Ford curves) To get correct brake points is strangely harder here than on any other track I drive. I know Nordschleife is longer but I guess it's because you have more "off camber corners" at Le mans... but also I haven't driven it as much as I have Nordschleife
Sachsenring.- Full of fast turns that i just cant seem to get right. I probably over drive the car which results in me mostly getting lots of understeer. Sonoma - the first corner is hit or miss. I usually mess up with the breaking point and go off track, the esses are another area i tend to not get right. Slovakia Ring- Im over all trash. except in the straits
I cannot get that first chicane at Monza to save my life....and after many years, I can now get all around Nordschliefe, not quickly, but I can finish a lap or two without hitting the Armco.....I dislike Indy, but strangely have had some real good races there.....Macau is nutty, but I quite enjoy it in WTCC...drifting through that fast right hander is puckering.
With most circuits I can get a good rhythm so there's not really a specific track that gives me nightmares, more so specific corners that I haven't quite figured out yet. For example, there's just something about the super long radius corners at Oschersleben that don't agree with me. Not sure if it's my setup or the patience of my right foot but I can never seem to find a consistent line which is also fast. Actually, now that I think about it, the first few hairpins at the Nurburgring are just...horrible. Feels so clumsy and even when I think I've done them well it still feels awkward.
Nordschleife and the Australian Nordschleife (Bathurst). I love them both and I've spent the most time on this two tracks. They are challenging and unforgiving. One mistake and you are done.
Pauil Richard is obviously a "love or hate it". For racing against others (AI or hjmans) it is very good (wide enough, many overtaking spots). For time attack, leaderboard it is too boring landscapre wise...