Just my personal top 10 favorites I have gotten alot of playtime out of:
Super Mario Land 1
Super Mario Land 2
Tetris
Dr. Mario
Kirby's Dream Land
Mickey's Dangerous Chase
Felix the Cat
Bugs Bunny Crazy Castle
Legend of Zelda - Link's Awakening
Motocross Maniacs
Really tough question, that for me boils down to Super Mario Bros. 2 or Super Mario 64. Both hold so much nostalgia for me it is impossible to pick between the two.