NV, hands down.
I'm actually not a big fan of the Fallout games outside of NV - I respect FO1+2, especially the atmosphere and willingness to be both dark and bizarre, but don't particularly enjoy their core gameplay loop. I respect FO3 for the gameplay loop it established, but don't really have a high opinion of its writing or atmosphere. I really don't like FO4. 76 is an abomination.
Strangely, though, I have a fondness for Fallout Tactics. AT EASE, INITIATE!