My vote goes to NV over Fallout 3, even though I enjoyed both.

@BLKCrystilMage's outline of NV narrative factional choices is spot-on and is a big part of what makes NV so compelling. So much so, that I've found myself wantingly replaying the game just to see how the endings played out (Caveat: I choose not to play the Caesar's Legion for-the-win option.) The various philosophical bents gave the narrative a BioShock feel to it.

Then there is all the allusions to 1960 Las Vegas personalities from Howard Hughes, Rat Pack (Dean Martin, Sammy Davis Jr, Joey Bishop, &c), Elvis, Steve Wynn and others that gave these characters a familiar déjà vu feel to them which added to the intrigue of the overall narrative.

However, one of the appealing aspects of the game play was the character dialogs. While most were somewhat straight-forward and uninteresting, there were several that were outrageously hilarious like some of the Camp McCarran sniper team members or the Power Ganger at Nipton, etc. Even Caesar's serious discussion of Hegelian Dialectics was worth the delay in game play.

The DLC packs were integrated into the main game in such a way as to pique one's interest into finding out what these obscure references were all about, with Old World Blues being a masterful whimsical diversion.