I wouldn't really be able to write a full list, however, I can clearly remember the level that gave me the most trouble being "Lemming About Town". Although not nearly to the same extent, it was somewhat similar to "Save Me" from the original, in that I didn't beat it until quite a long time after beating the rest of the game - and here too, it was due to execution more than due to solution. (There's a huge difference in how long we're talking though - I beat Lemming About Town a few weeks after I beat the rest of ONML, whereas Save Me remained unsolved for over 10 years for me.)