All-time classic, the way RPGs were meant to be played.
While I do appreciate linear dungeon crawls such as Icewind Dale or Neverwinter Nights and no doubt the D&D ruleset is a time-tested institution, there is something else to be said about Fallout's open-ended, free-roaming mode of play. It is one RPG where you truly feel like your actions and decisions are your own, and not some scripted plot you hack and slash your way through in the order that the developers intended. You might play through the game five times and get five different combinations of endings depending on your actions in the various towns.
The SPECIAL character creation system is also revolutionary in its own right, allowing a good degree of customization in making a variety of characters. There are no character classes per se, the scope of character creation limited only by your choices in picking traits, perks and abilities. Sharp-shooting doctor? Check. Diplomatic brawler? Why not.
If you've played Elder Scrolls IV : Oblivion recently and liked it (the ability to freely wander across the world map, not its graphics or combat system), chances are Fallout's brand of gameplay might interest you too. Also check out Arcanum, another excellent open-ended RPG that has sadly fallen as flat as its fellow title as a franchise.