It is the White Castle slider! At least according to Time. And the reason is the "now-iconic square patty" that debuted in 1921 in Wichita, Kan. was the first burger to spawn a fast-food empire.
It's been a while since we've had White Castles, but there are days when nothing hits the spot better than a few of these. And now I'll be craving some the rest of the day.