Psycho Fox Master System. Pure genius, greatest platformer ever made. I still have it and still play it often. So gutted that the 3D "revolution" has killed the 2D platformer, I haven't seen a 3D platformer thats beaten a really good 2D one.
NHLPA Hockey 93 Mega Drive. Not EA Hockey or NHLPA 94, 95 etc, it HAS to be 93! Still not been bettered in my opinion and I still play it often, probably the one game I could actually call my favourite of all time.
Quackshot Mega Drive. Second favourite, bit short but the graphics still look great today and its just so so good.
Alien Brigade Atari 7800. One of my first games (I had a 7800 for years before I upgraded to a Master System) and still holds up today, even if my gun fires slightly off to the left
Xevious Atari 7800. Favourite game for the 7800 era, bloody hard but great fun.