I only ever studied Ancient Greek, and that was kind of fun to learn. Mostly because there was plenty of fun things to read! Sex, violence, intrigue, plotting, battles, drug abuse - Homer has it all! Well, except rock and roll.
The language I found hardest to learn was Dutch. It's similar to both English and German, and the similarities utterly threw me. I found I'd end up speaking in either English or German, but in a bad Dutch accent :(