Paper Mario on N64, if you're talking about storyline completion. I don't think I'd ever be able to do everything on that game because there was so much to do. It's still one of my favorites. ;D
I think it was Super Mario Land 2: The Six Golden Coins. Super Mario Bros. 3 was probably a close second because I was playing that too. Super Mario Kart as well. Ah, the SNES era.