Sonic the Hedgehog
-- Extremely basic platforming when most platformers had long since advanced combined with overly-long boring levels with the only thing going for the game being the soundtrack and boss fights.
How to make any classic Sonic game an immensely better game? Boss Rush only, eliminating all the long boring stages between boss fights.
Close second: Mortal Kombat
-- Only became popular because it showed "blood" which was controversial at the time. Very mediocre fighter that, if it hadn't gained popularity through controversy, would likely have been forgotten just like majority of fighter games have been. Can you name even 10 separate fighter game series without having to look any of them up? Probably not. If it wasn't for the "blood" controversy back in the day, would you even know about Mortal Kombat? Probably not.
Come at me Sega fanboys. Face it -- the two most well-known games on the Genesis were nothing more than mere marketing ploys lacking any real gameplay merit, and popular only because Sega fanboys would buy up anything Sega marketed as "cool" because they're easily manipulated by Sega marketing departments and cared more about image than gameplay.
When you can't win the console wars by making better games, may as well take the Sega strategy of relying on marketing your games as "cool" and "edgy" and relying on a customer base who cares more about being seen as "cool" or "edgy" than actually playing fun games.
//Shots fired
//Battleship sunk