I just remember playing megaman 1 on NES over and over mostly to listen to the music after you beat the first bosses, i loved the music of that game so much!
The Super Mario Bros/Duck Hunt cartridge for the Nintendo Entertainment System was my first big console game (I played some crappy DOS games on Windows 3.11 before that), but I remember getting a SEGA Genesis with Sonic 2 when I was around 10 years old and being blown away by the graphics and game quality.
I had played Sonic 2 on the SEGA Game Gear since my 9th birthday, but I became a lifelong fan of Sonic the Hedgehog ever since I got to play the Genesis version (I'm 39 now).
I replayed that game so much, it's one of my core memories of my summer vacations as a kid.