I’ve gotta go with Megaman II. It was the first Megaman game that I played back when I was a little kid. I didn’t have my own NES, but I always played it at a friends house.
At the time, I appreciated the game for all the fun that it was to play, and paid little to no attention to the music. Nowdays, I’d rate the music as one of its greatest virtues, but then again, don’t all Megaman games have outstanding music.
But my personal favourite Megaman song is actually Crystal Snail Stage / Energen Crystal from Megaman X2, and not any from Megaman II.