I voted for Mega Man, because it's just about the most consistantly good series not made by Nintendo. The worst non-GB Mega Man is Mega Man 4, which was an incredibly dull ** title. Yawn. But there is no doubt that Mega Man 2 and 3 are among the best games ever made.
I likely would have voted for Street Fighter but I'm so incredibly awful at fighting games that my opinion of them is somewhat sour... it's just no fun to get your ass kicked all the time lol.