So, I've played all the modern Fallout games outside of 76, so I'm going to say Fallout 4.
[doges rotten tomatoes and vegies]
Hear me out, the old GURPS-inspired system (yeah, GURPS was going to be the RPG system that Fallout would use, but the black comedy and shit like someone getting executed on TV got GRUPS's creator to veto the proceedings, so we're left with a GURPS-lite that is the SPECIAL system) that was used up to New Vegas is a clunky mess that was good for its time but simply slowed the game down. It also helped that it had oh so many quality of life improvements over the New Vegas games, bringing the series to modern shooters. It also made power armor feel like walking fucking tanks, as the previous games just made them metal armor+ and that's it. You can also mod your weapons to fit your specific playstyle (and even name them), although semi-auto rifles have more perks to them than automatics. The graphics were pretty good, the environments weren't completely empty like New Vegas but not the same drab as Fallout 3, the combat is pretty effective given what it evolved from... I could go on here.
Sure it has its problems, but what game doesn't? Everything can be improved.