72 for me (at least I think it was 72). That's where I usually die, and even if I don't, I lose enough lives to die a couple levels later. My best level is still in the mid-70s, thanks to that level.
(this is of course assuming that parallel dimensions don't count... if they do, the hardest one that I have played was 51.4, I think)
Hardest boss battle is Kiwi Sappers, though Tagada Strawberries were also pretty tough before I figured out the best way to kill them. Sorbex Lemons are the most annoying, though they aren't exactly hard.
