This is the one that started the megaman francise (which dragged on for a long time, but was still fun).
The thing I always liked about megaman games was that they were so challenging and intense they blew your mind. the original set that standard. It is so challenging that by the end you'll be a game master. It was also ingenious in that you could choose who you battled, and stole their abilities when you beat them. It combines strategy and action.
Sadly, my copy of this game freezes after about half an hour into the game, so I've never actually beaten it. For those of you who have a working copy, keep it.