Thus far the three favorites I can remember off the top of my head are all quest names. The first quest with the title, "Ain't That a Kick in the Head?" seemed pretty fitting overall-considering the situation. "Nothin' But a Hound Dog," also, just because of the fact it was a quest for the King's dog and thus fit (also) pretty perfectly. Though the one I was most surprised by would have been, "We Will All Go Together." I suppose you could argue how much it worked for the quest, but I was extremely happy just to see a reference to a song by Tom Lehrer at all-I remember earlier while New Vegas was still developing there were suggestions. Once or twice people actually asked for one of his songs' to be in the game-and I supported the idea once I listened to them, but it wasn't something widely noticed by the board. So I wasn't expecting anything at all.
As a note, to those who don't know who Tom Lehrer is/his songs, there are two that I know of extremely fitting to the Fallout series. Considering it was made in the fifties. That would be the song referenced to, "http://www.youtube.com/watch?v=frAEmhqdLFs," and "http://www.youtube.com/watch?v=pklr0UD9eSo."