I was really contemplating putting Fallout 4 on here but it is *technically* a Fallout game so I did.
While Fallout 1 has the best MAIN story and Fallout Tactics has the best combat system, i prefer Fallout 2 overall. Bigger and better world, more factions, more quests, more replayability, more cult characters etc... Fallout New Vegas as a soft spot as it was the first time i actually played a Fallout sequel (played Fo2 before Fo1), but the gameplay is awful, imo.
NV probably has the best main quest out of any Fallout. All of act 1 is designed to make you know the region while act 2 has you do interesting content.
I have to say Fallout New Vegas, mostly because of my personal struggles with the gameplay in the older Fallouts