If you're not a zoomer play 1 and 2, 3 and New Vegas

If you are a Zoomer just play 4 and New Vegas since season 1 is heavily based off of nu-Fallout (4) and season 2 is leaning in hard to New Vegas

If you're a casual but want to GG at playing old school 1 and 2 (1 is legitimately great but 2 is over rated IMO but still decent) maybe look up a build guide so you don't run a character with dogshit stats

3 is overhated by people for some stupid writing decisions but has a better open world exploration aspect than NV and legitimately has good DLC's (all of which you should play since it looks like they're going to be leaning into some of the lore from some going forward)