i'd have to go with either nes or snes. i love to play all the classic nes games like contra, bubble bobble, and such, but i have no time to do so. also, i am quite the fan of the donkey kong series on snes. i think dk1 is the best of all.
I would say SNES (you might ask why) because 2D games are really good especially platform games like Mario 1 , 2 & All Stars DKC 1 & 2 (I don't like DKC 3 that much so) etc. you get the idea don't you
I must be honest. My favorite type of emulation are calculator emulators. They helped me so much when my TI-89 was out of battery power ;-) EXTREMELY HELPFUL