Sgt. Sykes said:
Mass Effect's Male Shepard. Holy dong, how did they end up picking this guy? FemShep and almost everyone else in ME gives great performance, but the main char male is just terrible.
The thing is, I don't know if it was a mistake in direction, or what, but Mark Meer is actually a pretty solid voice actor. Even in mass effect, he's the voice of a truly staggering number of the NPCs you encounter, and he does all of them well. Not sure what happened with the male shep, but I do agree it was an uneven performance at best... reminded me of The Avatar in Ultima IX. >.<
As for characters ruined by bad voice acting... well, I'm a veteran of the 90's anime OVA era... a time when tiny companies would round up homeless people off the street, pay them with what money they found in the change slot of a payphone, and then have them record voices for the characters... so my tolerance for bad voicework is probably quite a bit higher than many. XD
The lead character from "Samurai Legend: Musashi", that old Square-enix sequel to the PS1 classic, comes to mind though. Dear god, the voice actress sounded like Max Goof from "Goof Troop" doing a goofy impression.