No brainer really, The Legend of Zelda: Ocarina of Time and Pokemon Blue.
From the day it was released til I lost the cartridge around 2004, I probably played Zelda through at least twice a year. May not sound too impressive, but this was at a time where I didn't particularly care for games all that much. It still remains, to this day, my favorite game of all time, so much so I pretty much bought the 3DS for OoT 3D and the Limited Edition Zelda Console.
Pokemon Blue I played for a long time. I remember at 6-7 years old being so proud when I trained my Blastoise to level 100 and how upset I was when my step-cousin corrupted my save file and I lost all that work. I still played it off and on for years, I even remember the summer of 9th grade staying over at a friends house and we playing through together. Even now, I still prefer the sprites they used in the originals to the ones today.