Chrono Trigger
Super Metroid
Zelda: A Link to the Past
Secret of Mana
Super Mario World
Final Fantasy III (IV)

Turtles in Time and Goof Troop are extraordinarily great two-player co-op games, but not particularly fun in single player.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram