I know a lot of people prefer Fallout: New Vegas. And I'd never argue against them because it's a fool's errand to argue something as subjective as an opinion of a game. But I wasn't a long time fan of Fallout. Not that I didn't care for the old ones, I just didn't have a PC that would play modern games when Fallout and Fallout 2 came out. So I had no experience with Fallout before 3. And as such, this was my first experience with Fallout's post apocalyptic setting. The result was a game I played start to finish at least 7 times and up until the last playthrough I was still finding new interesting things I hadn't seen. This game also set the standard for DLC and what DLC could, and should, be.
Part of the thing for me was the callbacks to the cold war. I grew up in the 80s when the threat of nuclear war was palpable and very scary. It's somber tones really hit me. Where New Vegas is a game of society moving on, 3 showed us how bad it could have gotten. And it's psuedo 50s vibe coupled with cold war iconography was powerful and moving to me.