Of the modern Fallout games it is Fallout 4. Fallout News Vegas is great and I loved it and Fallout 3 was also a good game, but Fallout 4 is probably the best Fallout game and also one of the best games ever made.
The world building in Fallout 4 is amazing. This is one of the first games, where the world actually feels alive. You always have the impression, there is something out there, even when I am not involved. Guards getting attacked by dogs or feral ghouls. Factions fighting each other and NPCs reacting to your actions, outside of conversation and the story lines.
For example, if you clean out an enemy hideout, often after you finished Scavengers will show up. So the game reacts to you actions. This is something that happened during a lot of wars, that people were following the armies to collect leftovers from the battlefields.
Also you can walk into some great fights between the factions. My best experience was a battle between Children of the Atom, some Supermutants and a group of gunners, suddenly two Vertibirds from the Brotherhood joined the fight and a raider camp in proximity also tagged in. So the conflict became huge and it was more intense then even most World War shooters. And it all happened without me, I just joined in and afterwards after I left I still heard shooting, so it also got resolved without me.
Bethesda got it right with Fallout 4 and I hope they also manage to make the world of Fallout 76 feel alive, even when there are no players around and you are just alone with the world.