fallout 3, no question.
the whole vault idea at the begining was great, you had a charcter.
i didnt play fallout 1 or 2, but fallout 3 felt fresh, and fun, and had a sense of humour. at the time it was just the bees knees.
New vegas didnt have the same appeal, released years later, with the exact same graphics and a weaker storyline. Didnt even bother finishing it, i got bored.
didnt care for the "make a gun" thing or all the new guns, felt like fallout 3 was taken over by the COD team, same graphics, new guns.