The hardest levels are 1, 5, and 8 (I wouldn't know about the last two because I haven't managed to beat 8, yet xD )
I like the game though. I know it's been out for years now, but I wanted to make a comment anyways. I'm sure you've heard it a million times already, but I still think it's a fun game. My own frustrations aside, 4/5.
(My roommate's getting a kick out of listening to me freak out)