Mafia had awesome dialog. This game was a film (and a good film) in terms of its character interaction and development. I actually cared what happened to the people in it.

MGS series has some pretty good dialog but is let down slightly by the 'sillier' moments.
President Johnson: *grabs your crotch* you're, you're a man?

My favorite would have to be Grim Fandango though. This was classy, witty and very very noir.
Manny: "But you don't have any tendons."
Ballon Twister: "Yeah well you don't have a tongue, but that doesn't seem to shut you up now does it?"

Manny: "Run, you pigeons! It's Robert Frost!"

Manny: "I bet Hector pauses here every day, and thinks to himself, 'Boy, I wish I could see past my gut and enjoy my expensive monogrammed floor.' "