When I first heard they were bringing back Mega Man (and in 8-bit glory no less), I got so excited I pulled out all the old games and tried to beat them all. I even played Mega Man II on the difficult level, which I had never done before. Unfortunately, I didn't finish III in time, since Mega Man 9 showed up for download right in the middle of this run. It was a little jarring to try and finish up III while still trying to check out the new game. I have never even finished IV, but then, I got that one off Ebay and haven't had years to try and defeat it over and over like the others.
Mega Man 9 is the hardest one since at least the original Mega Man. Even after getting a few hints online, I got so frustrated I haven't played it in weeks. I'm going to play it right now though, as I'm strangely inspired for some reason....