Fallout 2, without a doubt.
I feel that the original is the "purest" of the series, which is to say, it's undiluted by any
direct predecessor, it has the most intense "Fallout-y" atmosphere, and the characters and storyline are more rich, stark and primordial than in any of the sequels.
However, Fallout 2 is longer, larger, gives you more toys to play with, vastly improves companion AI, offers more quests to accomplish, has cult classic references, is more "advlt," and its locations and important factions and characters are pretty damned cool.
So for me, Fallout 2 edges out the original.