Fallout 2 is probably my top game of all time, so that should answer which is best in the series as well. The original Fallout would be a close runner up, if I didn't consider them to be more like two parts of the same game. Kinda like Ultima 7. Technically there were two of them, each with an expansion of their own, but I still consider them two parts of the same game. Anyway, I'd actually put Fallout Tactics next in line, just because I think they got the Tactics part of it right, and even though the rest of it was a bit... off.
Now it's not that I don't like Fallout 3, because I do, but it's just not as good as the earlier ones were. Fallout / Fallout 2 absolutely blew my mind when they came out. Fallout 3 did not. It kept me busy for about a month, during which I admit staying up a lot later than I should have most nights, and then another week for each DLC, but I haven't gone back to play it again with a new character.