This is subjective there are things that some people like where as others do not.fallout 1 and 2 werent perfect nothing is, but fallout 2 was near perfect, at least for me.
In fallout 1 you had a time limit, you werent able to f*** around like you can in FNV and F3 in which you can play for years in game time without doing a single quest or talk to a single character... But then again you could just go in guns blazing kill everyone, even the children (which i never did

) it was all about player freedom and several interesting ways to complete quests, oh sure F3 had several ways to complete quest number 1 kill everyone number 2 do it peacefully...
In fallout 2 you could choose a side and stick with it or not choose a side at all and that was awesome it was about preservation of arroyo and that meant trying to save it any way you saw fit, it wasnt about puting a virus in a water purifier or not putting it in. And sex, oh yes you could even have sex, Mrs Bishop is one hot milf

...and wait...wait for it EVEN marry someone at the time that was an insane innovation.
Reading the dialogue was a pleasure, you had so many interesting characters and places from New Reno with the gangster feel, to Navaro and the Enclave to Vault City every place had a distinctive feeling to it.
The story was great it was simple,but detailed it was just a great adventure.
The gameplay elements were obviously awesome as well, getting a rocket launcher or a gatling laser late game was awesome.
And the atmosphere oh yes the atmosphere!
To me it was the best, a post apocalyptic game where you see children runing around and stealing to survive, the strange foods and drinks
The destroyed cars and buildings the sci fi enclave troops, the only state and government in the world the NCR, vault city a paradise rulled by a insane bitch

it was all great, so in the end for me 2 things were essential to the greatness of Fallout games.
Player freedom and atmosphere even though there were many things that made F1 and 2 great.
To end this rant, i still love Fallout 3 and FNV but the first 2 were the greatest.