First of all, Pokemon R/B/Y, because without nostalgia I don't think I could ever be able to play through them. If you look at them objectively, they're pretty bad, or at /most/ mediocre. The graphics are terrible even for the Game Boy, it's filled with glitches, the gameplay is broken, et cetera.
Then there's Star Fox 64/Lylat Wars; I have played that game so much that I can quote every level word-for-word. I love it so much..."Do a barrel roll!" "You're going to attack the enemy base? Great idea, Fox!" "Can't let you do that, Star Fox!" "Hey, Einstein, I'm on your side!" "Annoying bird! I am the great Leon!" "Never give up! Trust your instincts!" "Enemy shield analyzed." and all that junk. God, I hate how every other game in the series is mediocre to just /bad/.
Third, there's Super Smash Bros. I played that game so much when I was younger; it was the first console game we ever owned at my house. I remember my surprise when I completed 1P for the first time and unlocked Jigglypuff...man, those were the days.
The last game is, of course, Pokemon Snap. Because it's Pokemon Snap.