My favorite Mega Man game!
Fast forward some years, and now I have a Wii at home. When I saw the game was available, I immediately bought it, and it was like riding a bike; the knowledge of the enemies' patterns doesn't go away. I wasted Snake Man with the buster on my first try after more than 8 years. The robot master's weaknesses were still on my mind after a long time, but what I found the hardest was the Mega Man 2 robot masters (Doc Robot stages and bosses). I had to relearn how to beat them, or basically survive them. Quick Man in particular is extremely nasty.
If you want an old-school challenge, pick either this one or Mega Man 2, which most people say is better. Personally, having to beat the 8 robot masters twice, PLUS the previous game's masters without their weapons for the weakness cycle is a bit harder. And Mega Man's slide ability is awesome!