Fallout games (that I know of and have tried):
Fallout - Original and still worth playing, an excellent and fun game. Created the fallout world, made it a feature that you could hit an old lady in the crotch with a sledgehammer if you wanted, and made it so you'd have to deal with people being ****ed off that you did so. Always felt like one of the first genuinely grown up games I ever played (i.e. not aimed at kids, and not just a crap game with boobs in to make it seem 'adult')
Fallout 2 - IMO A better game, as the makers learned from the original but kept the 'feel' and sense of humour that made it great. Well worth playing through even if it's not aged well.
Fallout Tactics - non-RGP offshoot that... maybe isn't so great. At this point I'd say it's only for completists. It's not a great fallout game, and as a game of its own genre (turn-based tactical shooter) I'd rate it far worse than, for example, XCOM or the Jagged Alliance series.
Fallout 3 - An excellent game overall, but only a pretty good fallout game. Some decisions made in the setting and story ended up making it feel less like a fallout game and more like a tribute to fallout games. If you can get past that, the actual game is extremely good (and some people rate it higher than New Vegas). Had several pieces of DLC that were mostly meh. Also, making the player run through lots of near-identical subways was not my favourite design choice.
Fallout New Vegas - In my opinion the game FO3 should have been. It's 'more fallout-y', has interface and playablity improvements over FO3, better (IMO) characters and quests. Overall I feel (and I know a fair few people agree) that New Vegas is better than 3. It's not as cut-and-dried as some people would have you think, but as a lover of both I'd say NV is the winner. The DLC is also generally excellent, especially Dead Money and Old World Blues - proper quests, decent characters, interesting new weapons that aren't just "old gun +1" straight up replacements for everything. Marvellous.