Easy, the original Sonic the Hedgehog. It was the sole reason responsible for my drooling over the Sega Genesis when it first game out and I got a small taste at a friend's house. Who can forget their first experience creating a blue and red/white blur into a bottomless pit the first time after discovering just how fast that dang blue hedgehog can move? I played for days when I finally got my Genesis. Even today it remains one of my favorite games due to the overall nostalgic reasons and the fact that it's still freaking FUN compared to a lot of today's games. Simple gameplay, great sprites, and unforgettable levels (I still to this day have a special loathing of the Labyrinth Zone...) had me gripped for years since June 1991.