I define "classic" as "early 80s and before" so I'll say Space Invaders
I can't remember how long I've played this for in one "sitting" in an arcade, but it's a good reminder of how simplistic classic games are, and how you don't need a lot for something to be fun. I like the excitement of trying to get the last few spaceships.
To quote Futurama: "You needed to shoot where I was going to be" (probably butchered).