Mega Man 3 further refines the Mega Man formula with the addition of the slide, making movement a lot of fun. Other than that, it's really just more Mega Man which is great (although... maybe it's too much? the MM2 rematches are a bit unnecessary).

Also, this game introduces Proto Man and Rush so it deserves a great score.

If you want to try a classic Mega Man game, I recommend this one.