WARNING: SPOILERS FOR PORTAL 2 BELOW
Portal 2 is a pretty fun game and has pretty amusing dialogue. I kinda see it as a two-for-one deal: Game and Comedy. Out of all the dialogue, I think that Cave Johnson's parts are the best. As you've probably heard, his best scene is the following test sequence, where he talks about combustible lemons.
But all his stuff is pretty hilarious. Here's a link to an mp3 with all of Cave Johnson's lines (unfortunately not in order), as well as the new ending song.
On that note, I also enjoyed Jonathan Coulton's new ending song, "Want You Gone." Originally I wasn't so fond of it, but I have to admit that it grew on me a lot. I don't know if "Still Alive" is even a better song, or if it was just a better surprise.
By the way, if you're more a Wheatley guy, this link goes to voice clips of all the personality core lines: http://soundcloud.com/betarepeating/sets/portal-2-cores/