Oh, god. I can't even think of where to start. Punch Out, any Ninja Turtles game, especially 2 and onward If you see a game called Trog! snatch it up, I've been trying to find a copy for a while now. It was a sleeper hit. Any Mario, Mega Man or Zelda, of course. Final Fight
Honestly, I would just start grabbing cheap games by the handful and give them a try. There's a lot to discover and the prices are kind of random, a more expensive game isn't always a better one so just buy a lot of cheap ones.
Get Crash n' the Boys Street Challenge. It's got the characters from River City Ransom it it. Get that too. Get Uninvited, Dejavu, Shadowgate, Maniac Mansion, Double Dragon 2, SMB3, Mike Tyson's Punch Out, Solar Jetman, Elevator Action, and Zelda.