Put simply? It's my favorite game of all time and I probably have close to a thousand hours in it across multiple platforms.
I acknowledge every one of it's many flaws from the lacking of the Gamebryo Engine to the various lunacies of the story, but none of it bothers me.
No other game I have ever played has given me the feeling of atmosphere and exploration like Fallout 3 did. Ever moment in The Wasteland feels like a new experience. Every time I look off in a direction, I see new experiences to be discovered.
The music is perfect, the art design is perfect and the environments are incredible.
It has some of my favorite DLCs with The Pitt, Broken Steel and Point Lookout being absolutely brilliant in different ways.
And this was all just when I played through it a dozen times on console. When got my gaming PC, it only got more amazing with one of the strongest modding communities in gaming.
EDIT:
Since it seems a lot of people are bringing up the F3 v NV thing, I guess I'll throw in on that.
Objectively speaking, NV is a better game. Better written, better balanced and with more content. I do, however, still prefer Fallout 3. I think the main reason I still feel this way is the overpopulation of New Vegas. Everything feels so crowded with cites everywhere, so there's never that feeling that you can strike off in a direction and find adventure. In addition, NV has serious pacing issues. The first third of the game on the way to Vegas is essentially all combat and a character without combat skills has issues. Upon getting to Freeside, the game radically shifts and combat grinds to a total halt for numerous hours, at which point any character with poor diplomacy is left bumbling around failing dialog checks. The third after Freeside and meeting House, things get a lot better, but the stark contrast between the shooty bits and the talky bits is too extreme and jarring.
GloatingSwine said:
Fallout is a world which is rebuilding after the apocalypse, not huddling in its ruins like in Fallout 3.
I never understood why this is considered a fact. So often people like to say it's a post-post-apocalyptic series.
While this was certainly true for Fallout 2, Fallout 1 had an almost identical approach and feel as Fallout 3 with a heavily devastated world and a few small populations trying to survive in a hostile world.