I would say that most Fallout games haven't aged well and I don't think I would like them if I wasn't playing them with rose tinted glasses. Fallout 1 and 2 are classic RPGs. Isometric camera, minimal voice acting, and turn based combat, but if you are into that they are definitely worth it for the insane number of ways you can deal with the situations you get into and the different endings you can get. Fallout 3 is like Fallout 4, but with worse graphics and the worst gunplay that I've ever seen in a FPS. It's an action/adventure game with RPG elements, there aren't many decisions you can make that actually make a difference. It can still be a good bit of fun and is interesting, but the writing isn't the best and the PC port is a nightmare to get running on modern hardware. New Vegas kinda fixes the problems from 3. Better writing around factions and companion characters, more choices that actually have an affect on the world and the ending you get, and some of the best DLC I've ever played in a game. But it is far from perfect, it still feels awful to shoot a gun and empty locations really show off the short production cycle of the game, but NV is easier to get running than 3.

Personally, in my extremely biased opnion, I'd recommend New Vegas. Less finagling required to get running than the others and it actually feeling like the skills you pick make more of a difference than "do you want pew-pew guns, zappy guns, or do you prefer things that go kaboom?" It's $10 on steam and pretty regularly goes on sale for $5 (used to go on sale for $2.50, but hasn't since the Fallout show dropped)