The quest with retrieving the keys for that ghoul in FO3 was better than anything I've experienced in NV. The one with the android was also pretty good. Plus Reilly's Rangers... though it quickly devolves in a slaughter-fest, it's pretty nice how the quest triggers when you're wandering around in a ruined Pennsylvania Avenue.
There's also that big unmarked quest with the journals that eventually leads to the MIRV.
As already mentioned, NV has so much filler that the good quests get completely lost in the pack. Frankly, I can only remember the companion quests... and that horrible one that sent you to all the Ranger outposts. Twice.