It depends. Both New Vegas and 3 are pretty good. If you want a game with free mods from new outfits, to absurd amount of rapid fire, to be a playable Deathclaw, then I'd go for Fallout 4. Also, what console do you have?
@ExperienceDropOut Well, try all of them on PC. Add some mods in, try it out! I've played Fallout 4 with mods and I use Lightsabers of my own creation. And all of this is on Xbox One S. 😉👍
@ExperienceDropOut It's not hard, but be sure of how many you download. Depending on the mods, files can be very small or quite large. And it shouldn't affect your game, in terms of corruption.