Either British or the refined Australian accent. (Conversely the most annoying accent to me is the broad Australian one coupled with an over-use of Aussie colloquialisms like "Caaaaant, Maaaaaate (has to be nasal and drawn out like they're from the mid east coast, so it sounds like they're always asking a question), faaaarrk and whatnot.
Most hilarious is probably German. Learning it in highschool ruined that language for me. In our listening test we were treated to a "German woman" "waking up early": "Ohhhhh!? Zechs! ZECHS! (means 6 as in 6 am. Yes it does sound a bit like that other word, hence our teacher was left yelling that we were all immature as 20 students burst out laughing in the middle of a test)