I can't really think of any Final Fantasy game that had terrible voice acting so its hard to pick a worse. I suppose I'll pick X; nothing against X, but there were a few voices that irked me just a little. The prime examples are Seymour and Rikku, their voices bugged me a little, and it didn't help that I hate the characters themselves as well. However, there is some voicing I do like, Auron's voice was very fitting to the character and all around badass. I also like Wakka's despite his weird speech, it also helps that its the same guy who does Bender from futurama; and Lulu's was pretty good.

But all in all I have to say X was the one that I had the most problems with, but that doesn't mean I didn't think it was well done.

The best voicing in the series was XII by far. I wouldn't at all be surprised to see most of the people here pick XII. The voice acting was very professionally done, you can tell the actors cared about what they did and they sounded convincing in the game. It was also awesome to add different accents to the characters; also to give them different variations of the english language, and not just straight american english that you hear with most dubbing in games. They really went out on a limb to make sure the voicing in this game was quality stuff and it shows