I have all four Fallouts, and all the games have been a blast, the best RPGs I have ever played.
Fallout 3: This was the first fallout I played, and the first western RPG. I was taken aback by the complexity, but as I got used to it I was hooked. Spending hours scavenging for junk to sell should not be fun in any context, but it is in Fallout 3. The only problems I have are the story, which is badly written and feels like B-movie pulp, the character designs, which look like talking mannequins, and the dialogue, which is simplistic as hell. Awesome game.
Fallout 1: The first game I bought on my Grand Old Games Account. I like the atmosphere and the combat system, and the story, hate that damn timer in the beginning. This is a sand-box game, people play it for the freedom it has, we don't want timers in our game. Good, but not great
Fallout 2: Now this game is way better than 1 and 3! Everything about it is perfect: the combat, the character building, the story, the funny if juvenal dialogue. You can do anything, become a pro boxer, a pornstar, slave trader, sheriff, kung fu master, a master thief, a spy, anything. Of the first three games, I still play F2.
Fallout New Vegas: Now this may be the best of the Fallouts,tieing with Fallout 2. New Vegas took all the major flaws of 3 and streamlined it, the character building wasn't as broken and it isn't as easy to build a powerhouse, the dialogue makes sense, the graphics have improved, the combat is fun and not wooden, the story is interesting and entertaining, and there is no cap at level 20!
If I had to pick a game for a noob, I would pick Fallout New Vegas. It has the best part of the original two Fallouts and the innovations of Fallout 3.