I'm stuck at my traditional part: Wily himself.
No seriously, for almost every MM game in the classic series I can get all the way up to wily himself, defeat his first form then die only to comeback and try to defeat him with just a megabuster because all the good weapon energy has been depleted. In my 20 years of gaming i've beat every classic MM game at least once, but it always took a few years because after so many game overs i'd give up, and not come back to it for some time.. MM9 is no different and for that I love it.
it sits there, on my wii menu, taunting me.. but i know in another year I'll go back and miraculously pull it off :P