Mega Man 2. One of the first games I owned for the NES. Got in on Christmas day when that was the latest Mega Man game. I love the original series in general, but nothing has been able to top that game for me in the Mega Man universe. It's just so damn well designed (no other Mega Man lets you take down the bosses the way this game does with multiple weapons working in different ways against them), and has the series best music.