Really, this is difficult to say actually as most of the games have pretty VA work, even X is mostly good with only a few key characters actually having annoying VA work (Yuna and Seymour).

XII is probably the best overall cause as Bolivar said Gabranth just has this awesome voice and the fact they added in the muffled voice to compensate for the helmets was just great as well as the sheer number of accents and dialects used to represent different places. From what I've heard of the original Japanese, I might actually say that the English version is overall superior.

Worst? Either Dissidia or the Compilation of VII. Half of Dissidia's VA is awesome and the other half is either acceptable or just atrocious... The Compilation also has some pretty bad choses when it comes to voice work as Bolivar pointed out with Cid suddenly becoming a southern hick... Steve Burton as Cloud is also hit or miss, in some titles he does a great job (Crisis Core/Kingdom Hearts) and other times he sounds like he's forced to be there and just phoned in his work (Dissidia).