Startropics for NES. That game was frickin awesome, but I've run into very few people that ever played it. Vectorman for Genesis seemed to widely promoted, but I never ran into anyone else that ever played it.

This is my own personal pet peeve, but Aggressive Inline for PS2. Back in the day when Tony Hawk was still halfway popular and they were releasing X-TREME PRO everything, they released an inline skating game called Aggressive Inline. It was the first game to do away with 2:00 time limits, have really interactive levels, feature the now standard "find this guy and get your next challenge" open world style of play, and a million other now standard gameplay points. It was nigh on flawless, except for the simple fact that it was rollerblading and rollerblading was not cool anymore by this point. As such, no one bought it, Tony Hawk ripped EVERYTHING off from it (though it took them two games and several years) and then everyone else ripped off Tony Hawk again.