I would like to say Ms.Pacman or Mario Kart 64 but it's probably Fortnite overall & I say that as someone who has never played Fortnite but it's clearly has had these kids attention way longer than our favorite games kept our attention.
Years ago (1996 to 2016) it would have been Team fortress but in the last 10 years I have played Overwatch on and off through the good and bad but there is just something about class based shooters that I just love and I inevitably keep coming back to them and Overwatch has been a constant in my gaming pretty much since launch.