I think each fallout game had its up and downs.
Fallout1 was a very solid game you could beat it with any type of character if you could figure out how to play it right. Only thing I didn't like was the waterchip timer.
Fallout 2 was a bit more goofy had more humor to it and pop culture references. It had a few broken and out of place quests and stories. I really could not say it was better than fallout 1 but adding in the new weapons and the car was a major plus from fallout 1.
Fallout 3: The main quest wasn't exactly the greatest plot I've experienced playing a video game. Broken steel made the ending alot better. Overall I think it was a good game the add-ons made it more interesting.
Fallout tactics: THis was a cool game I never finished though so I don't have much speculation on this one when deciding which one is my favorite.
Fallout New Vegas. I don't know what to make of this game. It has some cool new features that weren't in fallout 3. The writing and main quest was alot better than fallout 3. I'd have to admit its an okay game I kind of liked fallout 3 a tad bit better for some reason I don't know what it is exactly...
I really couldn't pick one as favoring one over the other I liked all these games but each game had things I didn't like and somethings I did like. I think if they weeded out the good and bad from these games they could make the greatest fallout of all time, until then i don't have a favorite yet. I do have to say that the fallout series is my all-time favorite series/franchise of video games. Each game to me had a void a feeling of something lacking and I don't know what it is.
If I really really had to pick one I'd say fallout 2. And thats if I had to pick one, like I said I really have a hard time picking one as a favorite over the other.