Fallout 4 is, far and away, the best "game" of the modern ones. It feels much better to play in almost every way than the other ones. Especially the combat. There's some interesting stuff in it, but it's largely the mechanics that keep you coming back, not the RPG or world.
Fallout 3 has perhaps the better realized world out of them all; the way it all fits together is great and there's a lot of rewarding exploration in it.
Fallout: New Vegas is, far and away, the best Fallout game...it harkens back to the roots way more and is the best RPG -- by a long shot -- of the 3.
Obviously YMMV and others will feel differently, but that's how I've parsed out this series so far.