Dead Money's entire story is pretty much a single questline, right?

On the first playthrough the radios annoy you and the ghost people spook you out so you probably just want to leave that place, but playing it a second time and actually paying attention to not only the characters but the story that your surroundings tell you in the Sierra Madre, and you can really begin to appreciate it.

Getting the happiest ending is really satisfying.

'I forgot to remember to forget' i think because Boone is awesome, but really shows a side of Boone you don't get to see often. Also you get to blow somebody's head off but that's aside the point. Really for me its NCR, Boone, then maybe some innocent people.

I can't remember what the name of the quest is but its the one where you have to help the boomers

The entire Old World Blues

Oh, and in the vanilla game I'd have to say the quest where you either assassinate or protect President Kimball. There's so many possibilities, there's a sense of tension and there's some funny dialogue as well.

Dead money and lonesome road quest lines are some of the best written out of the entire series imo. They give a very intense somber feeling and are the most apocalyptic areas in new vegas. I also count someguy's quests as an unofficial add-on that is also in my personal headcanon, and they are really well written.

lol if mod quest, one of the most funniest, and challenging was The someguy quest " The Better Angels"