Fallout

I'll admit one important reason is just that it was the first I played. I'd never played an open world game before so the revelation of even the possibility of there being so much depth and freedom was amazing. That said, I also think it is generally better in other ways like setup and writing.

It also had the best soundtrack. There are people who listen to the F3 radio outside of the game. I don't think I've ever heard anyone say they loved the other soundtracks at all, let alone enough to listen to them repeatedly.