I don't have a particular level I hate, but 43 has got me killed more times. I knock the lemons down two levels at a time until all three are on the bottom and try to kill them all at once, but I consistently forget that the lemon bombs are coming from top and bottom. I get confused, and then I get killed

Also, Level 79. I can never quite remember how to navigate since it takes me over a week to ever get back there.
As far as PDs go, I hate 64.0. Hate it. HATE IT! It took me for freakin ever to get my Rigor Dangerous key off PD 55; I can never react in time to manage the ice levels.
On a side note, I got my first Hearts card today. Biiiig win for me.
