Fallout: New Vegas
Fallout
Fallout 2
Fallout 3
Fallout Tactics
Fallout 76
Fallout 4
Never had the displeasure of playing BoS.
My reasons for playing New Vegas at the top are somewhat complicated. New Vegas on the one hand is inherently limited as an RPG compared to 1 and 2 because it's basically impossible for it to replicate PnP tabletop as well as the Black Isle games do, and characters are far less stat and roll dependent than they are in 1 and 2, that's an inherent negative for me. However that being said, New Vegas surpasses 1 and 2 in terms of narrative roleplaying. In Vegas choices are more plentiful, your character is more mouldable (and IMO I had the easiest time actually roleplaying), Skills across the board are used more and in more interesting contextual fashions (You can definitely see the planned development philosophy from Van Buren bleed into NV here), and your investment in weapon skill is more diverse. You aren't pigeonholed into the same kind of build as tended to happen in F1/F2.
I also think worldbuilding in Vegas is the epitome of the Fallout setting. Fallout 1 was very well executed but it was concise and leaned more on the pulp sci-fi, Fallout 2 presented a lot of really good ideas and brought the setting to an interesting point but it was the tonal equivalent of Homer Simpson's makeup gun. Just too much, inconsistent and all over the fucking place. Vegas takes the interesting ideas of Fallout 2, introduces its own and builds upon the setting but has the tonal/thematic cohesion and confidence of Fallout 1. Namely stuff like the tribes and the concept of post-nuclear tribalistic culture, both in the Mojave with its more non-literal forms and in Honest Hearts's Zion with the more Fallout 2 esque tent-dwellers. Or the Americana mysticism, or the pulp sci-fi dark comedy. It also injects a spaghetti western feel which is something I love because I adore westerns.
3 goes above Tactics because I enjoyed the gameplay of it more than Tactics, and I think the Midwestern Brotherhood of Steel is quite literally just as much of a bastardization as the Capital Wasteland chapter. Fallout 3 had worse lows than Tactics, but there were at least times in Fallout 3 where I enjoyed exploring a bleak post-atomic dungeon and listening to the ambient tracks. Tactics was just a deluge of sub-par mediocre sludge with a number of irritations.
76 goes above 4 because no story is better than one that actively irritates me.