I think USF has the worst because the person(s) sound so whiny. It also sounds like they are yelling all the time.
I think the best voice acting is probably Ostheer (although COH2 isn't going to get any awards for sound). At least the Ostheer sounds are "dignified".
Hah this is funny. I voted the opposite of you and when I read your post discovered my reasons are the same - only in reverse.
I guess this means it's hard to please everyone. Maybe different areas of the world are a factor even within the same language.