New Vegas and 2 are the obvious choices, but I’m going to go out on a limb and say that 76 is third. Speaking as someone who didn’t enjoy 3 and 4, 76 fixes a lot of my gripes with those two games while simultaneously having some of the most creative world building in the series. The game taking place shortly after the bombs dropped makes Bethesda’s heavy focus on pre war America fit a lot nicer than everyone constantly talking about a war that happened 2 centuries ago in 3 and 4. The various factions, from the Responders to Foundation to the Cult of the Mothman, are creative and have a lot more depth than any faction or settlement in 3 or 4, and feel a lot more interconnected in the world. Appalachia is also just really awesome to explore

Fallout 4 is my favorite followed by New Vegas. The gunplay and relative ease of moddability has me at over 1,600 hours in-game, despite all its issues, the less-than-impressive story, and the like. NV somehow feels a little sparse in comparison to even FO3, but the storyline and dialogue more than make up for it. Plus wild wasteland, of course, which should be in EVERY Fallout game imo.

For some reason 4 is cripplingly addictive for me every time I pick it up but I can’t get into any of the others

I would probably agree. Still has a pretty active modding community which is nice.

Fallout 3 has a nostalgic place in my heart and Fallout 4’s new mechanics and settlement building were a fair amount of fun though

I really like the first game. Talking the master out of their plan is incredible and the game world is so interesting to discover. I learned a lot of new English words from it, and because the characters talk with slow text on the screen it is easy to follow compared with speech.