hmm this is tough to gauge for sure but I think probably Super Mario Bros. Yeah cause you could never save on NES so always had to play all the way through each time. It was nice on Super Mario Allstars on SNES when you could save after each world at least.
Runescape, I played that game every chance I could as a lad. Now adays I play it everyonce and a while but not even close to as much as I used too. Many people (everyone?) on these forums dislike Runescape but it has grown on me.