Just what the title says. I know New Vegas is the fan favorite (and it's mine as well), but what do you personally like replaying the most? Is it Fallout 1, with its tight narrative and quick replay value? Is it 2, with its massive expansion on Fallout 1's formula? Is it 3, with the most atmospheric Wasteland to explore? Is it New Vegas, with the best roleplaying and replayability in the series? Is it Fallout 4, with the best gunplay and survival mode? What are your thoughts?
You should try A World of Less Pain for New Vegas! Lots of fun if you want to combine the aspects of loot and shoot in 4 with the roleplaying and world of FNV. Another Millenia also adds a bunch of uniques to find.