Megaman 2, Super Mario Brothers 3, Megaman X, Super Mario World
The best Sidescrollers ever made
Chrono Trigger, Final Fantasy VI and Earthbound
These are games that are hundreds of times better than every JRPG made after than. Everything from the story, the characters, the atmosphere and the music is almost perfect.
Super Metroid and Castlevania Symphonyof the Night
These games are the Standard to 2D exploration games. The gameplay, the Power Ups, the enemies and the secrets are tall crafted with perfection.
Doom 2, Half Life, Duke Nukem 3D, Blood, Goldeneye
Some of the best First Person Shooters ever made. These games put newer FPS's like Cod and Battlefield to shame.
System Shock 2, Deus Ex, Thief 2, Silent Hill 2 and Prince of Persia: Sands of Times
These are the best of its kind: System Shock 2 and Deus Ex are the best First Persons RPG ever created, Thief 2 manages to be the most open and fun Stealth game ever made, Silent Hill 2 is probably the best written and most scary game ever and PoP: Sands of Times is the best 3D platfformer ever created.
Half Life 2
This game is Eight years old and it manages to surpass almost every new game in terms of gameplay, immersion, art direction, characters and story. And this isn't nostalgia, I bought the Orange Box in April 2011 and thought it was better than 95% off the games released in the last 7 years