I think if you're going to have story and voice acting in a game, no matter how crappy or great it is, voice actors make or break it for me. A great voice actor can pull you into a great story, or know how to make up for crap writing. Which voice actors do you think did particularly great jobs in games? A few of mine (omitted all Mass Effect actors because I could turn that into an essay):

Andre Sogliuzzo as Ethan Thomas in Condemned 2 is kind of a favorite. The script seems like they wanted to write Ethan as your typical vulgar tough-guy, but Andre went a different route and delivered his lines like more of a cynical dick who was tired of being screwed around with.
Other notable VA roles: Male Khajits in Skyrim.

Joel Watson as Pvt. Garnett and [uncredited] as Pvt. Allen in the Brothers in Arms games. The first two games had wonky voice acting, but the VAs for Larry and Garnett did a great job. They made the characters funny and likable, which is a real kick in the stomach when you know what happens in the game.

Anthony Di Longis as General Serrano in Bulletstorm. Yeah, yeah- Blum and Hale did their usual great job being Blum and Hale, but Di Longis was a huge surprise. He played such a great evil asshole, and the fact that you're supposed to keep him alive for a huge chunk of the game while he taunted you made me just hate him as a villain even more. Definitely goes down as one of the best evil laughs I've ever heard.
Other roles you may know: Marshal Leigh Johnson from Red Dead Redemption.

Syd Lexia wrote:

I'm a big fan of Troy Baker as Vincent Brooks in Catherine.

Troy Baker always does a fantastic job. I know I gush about Brothers in Arms enough, but that was another really good performance he did.

Dimaggio always does a great job, but he did give Marcus a lot more range than you'd think someone would give to the character.

One I forgot is the voice actor for Khan (can't find credits for his VA) in Metro 2033. He made what was already a creepy section even creepier.