Nevermania 6 was very hard to get past at first, but once I had gotten the technique it was no problem.
Nevermania 7 was one of the hard ones, but its not as hard as the other Nevermania levels.
I think a lot of people get stuck at Nevermania 8, (Because of the annoying goal) which is why the later Nevermania levels are not being brought up as much.
Nevermania 9 is definitely one of the hardest.
I have no idea how someone could finish Nevermania 11 using the last cargo pick up, it's insane. It would be one of the hardest levels if it was not possible through some other method.
I'm stuck on Nevermania 13 at the moment, I think i can get it, i'm still not sure if it is as hard as Nevermania 9.
I'm not sure about the other Nevermania levels but they look hard and noting the lack of replays they probably are the hardest levels.
Retour De Force 20 and 14 deserve mentions, they were the only non-Nevermania that were really tough.
Retour De Force V looks really hard, but i have not been able to unlock it.
I havn't played all the non-official sets, so there might be some other really hard ones out there.
Btw, this is just counting levels that are difficult to finish. Not coin gathering or fast finishes. The levels i mentioned are still probably the hardest in these areas, but i would add Tour De Force 3 for collecting all the coins and probably all of the other Nevermania levels for all coins and fast finishes.
(I'm not including levels like Retour De Force 17, where you probably cannot collect all the coins)