3. Mega Man 2
So I realize I've been writing a lot about the music in my top games, but that has certainly been a large part of why these games latched themselves onto my child self's brain and stayed there for so long. Mega Man 2 probably has the best music of any NES game (of course, my opinion, just like this entire list). And that intro scene with the simple scrolling up of the skyscraper to show Mega man looking like a bad ass at the top while the “rock out” music plays hooked me from the very beginning. Combined with the awesome and true to gameplay box art (sarcasm), I didn't even need to actually play the game to like it (sarcasm?)!
Thankfully, it's not just the music that is good here. I was finally at an age where I began to grasp the “certain boss weapons are strong versus certain bosses” concept and really started to own those poor hapless robot bosses. The platforming was still just as devious as Mega Man 1 (How many times did I die at Quick Man's stage before I discovered the “cheat” of using Flash Man's technique to stop time?), but the game actually had difficulty settings so youngins could beat it without resorting to one of those Game Genie contraptions.
On a final note, remember how awesome it was at the beginning of Dr. Wily's stage when he comes down in that UFO and then you see his epic castle and then the epic music plays and the you have to jump across blocks out in space while a robot dragon chases you and then you have to kill it? Epic!