It's tough because there's so few of them. I would say XII had the best and X had the worst, but I didn't have any problems with it.

The best voiced character might be Judge Magister Gabranth... Balthier and Ashe (don't hate me) come close though... no wait, all the judges were awesome.

Worst? Hmmmmm.... maybe Cid from FFVI: Dirge of Cerberus. Didn't they just make him some hick?