Some of the CE's ya'll have had trouble with I found fairly easy. Le Mans is one of them, sector 1 wasn't terrible, 2, 3, and 4 were easy, the final should be oh grawlix is going on here!? That last sector has always been surprisingly difficult IMHO. I tried it again yesterday, golded everything except that, and ran out of time before trying the full lap again.
I had a crazy difficult time with Sardegna also, and Spa to an extent.
Now if I had to choose a "final boss" race, it would either be the Spa 1 hour due to it being the longest World Circuits event or the 1 hour Maggiore race because it's the last mission in numerical order.
The most difficult race, IMO, is one of three: the Tokyo East WTC600, the World Rally Challenge at Eiger Norwand, or the 1 hour endurance race at Maggiore. I have yet to win the Eiger Norwand race or the Maggiore race at maximum difficulty (technically Maggiore only has one). Also a Group 3 BoP race at Mount Panorama caused a few people some trouble a while back. I thought it was easy myself, but this is coming from someone who's hard pressed to find words to describe the Maggiore race that won't result in a ban.
For the record the most difficult track I have ever seen in any racing game is a mess of right angle jump,-turn combos called Spilshinanke. It's run in anti grav ships. Once mastered, though, it's a lot of fun!