Interesting thread...
For my LemEdit packs, all of them are the tenth levels, except 6, where I think 9 in harder, and 9, where I also think 9 is harder, unless you've never seen the solution to 10, which I doubt.
Overall, in LemEdit, my hardest level is still, I think, "4 Pixels (or so) from Victory" (Set 3, level 10).
In my Cheapo sets:
Do the Mario!: I don't think any of the levels are truly hard, but I like "
??".
MPT: The Bar of Fire, just for its downright annoying precision. That, and it is pretty much impossible before you know the trick, even when you DO know it.
MS Paint Adventure: Ramico's Revenge, doubtlessly.
IS World: The last Ramico level is hard... but Fall and no life 3 is also tricky. I don't know.
I found Return of the T quite easy, personally. And, just looking at it, I don't see exactly what can be so hard with "Lemmerixo", but as I say that, I haven't beaten it yet.