as someone with the honor of having played every (video)game in the franchise, I can say with certainty it’s Fallout Shelter Online. Fallout just ain’t right without dragons. /j

but honestly, it’s gotta be new vegas for me. kind of a boring answer, but it’s the game that truly made me a fan of the series, and introduced me to the modding scene. I wouldn’t be the same if I had not played it.