There are a few contenders for me in "Worst Voice Acting I've ever heard".
The top two in my mind, though, are Star Ocean: The Last Hope and Baten Kaitos: Eternal Wings and the Lost Ocean. And for the non-JRPG school, Battlestations: Midway and Battlestations: Pacific had dreadful voice acting. The voice acting for both the American and Japanese sides was horrible.
- - - Updated - - -You know, I'm aware that what is 'good' or 'bad' is subjective, but come on! Vaas was an excellently performed character!