Fallout 2 and New Vegas are the better two IMO; made by the same group of people.
FO3 has the best setting and some fun side quests; but the main story is dull and the game play is inferior to New Vegas.
New Vegas and FO3 are adventure games much like FO4, the meat of the game is in exploring. They can be modded (
http://www.nexusmods.com/games/?) to look as good if not better than FO4 if you have good hardware.
FO3 has some issues with the new windows and you may need some 3rd party patches.
FO2 is isometric and is more of a traditional story driven American RPG like Baldur's Gate.
FO1&2 may need patches and mods to run well on newer systems, see
http://www.nma-fallout.com/downloads.php for details