I absolutely adore the Fallout series, so much so that I own Fallout: Brotherhood of Steel (for which my soul will burn in eternal hellfire), but anyways here is how I would rank the games in the fallout series
I consider Fallout 2 to be the best game in series by far, even with all the bugs not all of which are a detriment to the game (like the having the wandering luggauge compartment save my life on many occasions) and various patching issues. It has a great story, a certain charm and a dark sense of humour which I have yet to find in any other series, and as such I can't help not loving it.
Fallout 1 takes my second place; in small part because it is the first in the serious but mostly because it has a tight story arc and quest progression, and finally but not least because of The Master ("join. die. join. die." being my all time favourite villian speech ever).
Third place goes to Fallout Tactics: Brotherhood of Steel, if for no other reason than it introduced the "hawk" power armour and the gauss minigun. It was also the first fallout game that was radically different from Fallout 1 and 2 with its innovative CTB (Continous Turn Based) system.
Fourth place goes to Fallout 3, and my sentiments were pretty much described by
It's just a very bad sequel.
The DLC helps to improve the game but ultimately if you just explore a bit you'll hit the level cap before having completed even a quarter of the game(something which comes as second nature to any fans of the originals). What I found the most lacking however, was the sense of humour present in the other games.
And taking last place is Fallout: Brotherhood of Steel, the main reasons being the lack of character creation,
the skill trees ripped directly from the baldurs gate console games(which are almost equally bad), shoddy gameplay, an interesting if rather weak story and last but not least your soul being damned to eternal hellfire for owning it.
I'm very much looking forward to New Vegas and fervently hope it will be good game, worthy of the fallout name.