Ok, I'll get in this one too. I didn't play and am not interested in Fallout 3 or Fallout 4.(for various reasons that I won't go into)
I did play Fallout 1 and Fallout 2, and recently picked up Fallout New Vegas.
1. Fallout (the original) I remember that I really liked this game, it had great atmosphere and setting and story.
2. Fallout 2 - I didn't finish this game either, like some others who responded. I thought it was a letdown compared to the first one, and that they put too much wacky humor into this one, as opposed to the more "pure" experience of Fallout.
Fallout New Vegas - I spent around an hour playing it so far, and I can tell already I think it's a great game. I love the desert atmosphere, and they got all the details right, and it shows in the graphics. (I have spent a lot of time in the Mojave desert on vacations, believe it or not, however odd that may sound).
I can't properly rate it yet, but I can tell already I would put it at least as my second favorite, and maybe even has a shot at replacing Fallout as my favorite of the series.