Personally, I managed to get my hands on Fallouts 1 through 3 and New Vegas and found them all to be fairly decent in different ways.
I thought Fallouts 1 and 2 were pretty good, but then I still haven't finished either one mianly because (in my opinion at least) they both have this arbitary habit of not telling you crud.
The first game was particular guilty of this because it gives you a limited amount of days to complete the main quest, seriously, who puts a time limit on an RPG's main quest? this is not helped by the fact you go to the Vault marked on your map by the Overseer to find the Water Chip only to find it nowhere and given no clues at all where to go next.
The second did alot better, it seemed to tailor quests to your abilites pretty well, but there are still afew things I wish the game would give the common courtesy of explaining, I had to consult a game website just to find out I'm supposed to blast open a door in the Arroyo Temple and where I'm supposed to find a time bomb in s specific spot. Edit: on top of that, it doesn't give you a time limit like the first Fallout did, that alone I think makes it superior to the first,
I suppose an actual tutorial would be too much to ask from those 2 games.
Anyway, Fallout 3 was my introduction to the francise, and I thought it was fantastic. Granted, the combat gets too easy in later levels, but the game did a great job immersing me into the experiance of exploring a post-apocolyptic America, the harshness, the drama, Bethesda did it's best to create a fascinatingly dark world out of Virginia and made it a great (if depressing) experiance to be here.
Fallout New Vegas made alot of gameplay improvements over Fallout 3, especially in the combat, and it had pretty good writing and characters to go with it, but I felt it messed up the open-world part of the game early on because of how it's nearly impossible to go north from Goodsprings because Obsidian decided putting Deathclaws in the I-15 route and forcing you to loop south to past them is a good idea to put in an RPG Sandbox, Project much?
Another thing is that I found the game harder to immerse myself in then I did Fallout 3, which was disappointing to me 'cause the immersion was what I liked about Fallout 3 so much, and when I first learned about F:NV I thought i might get a similar experiance.
The only other problem I had was with the loading times, I had to put up with a buttload more loading times than I did Fallout 3, which I felt broke the flow of the game quite abit. I know most players complained about glitches, but while I have faced afew of them in my play throughs, I didn't get it as often as I kept hearing other players have.
Overall, I don't really get why the more Veteran fans of Fallout hate the Bethesda Fallouts, I found all the Fallout Games to be decent in their own way, but by the same token, I found they also have problems of their own, with some parts of one game being better than others.