I was in a discussion with my brother about this subject and he kept sayin the metal gear games. I strongly disagreed because I always thought the voice acting in those games were overrated. I feel that the game that has the best voice acting I've heard so far is Heavenly Sword 

Thought King Bohan's voice was awesome, I would have to agree with your brother, MGS gets my vote.

I still love San Andreas quite a bit on voice acting, but I'm probably in a minority on that one. I mean Samuel L Jackson and James Woods? Easily some of the best voice acting in video games even after all this time imo.

David Hayter as snake is the best voice actor in my opinion.

Mass Effect

the dude who did bohans voice(i dont know his name)>>>david hayter by far!! in my opinion

Half-Life series, espesically Half-Life 2. Excellent.

Portal's voice acting (all done by one woman) is good too, and TF2's is cartoonish but good as well.

The voice acting in the original Resident Evil is so bad it's good.

Aside from that, I'd say Mass Effect. But then again, I may be biased.

For me I would have to say Knights of the Old Republic, followed by Lost Odyssey, and Mass Effect. Bastilla, Carth, Mission, Julani, Jansen, and Wrex. Were all wonderfully voiced characters.