I love the MM9 Stages.It also seems like Hornet Man had a pretty tough stage without items, but it's been awhile.
They are long as hell, play great music, and the Difficulty Level is pretty much perfect.
protip: Try Pharaoh Shot and keep a charged shot above your head.For myself I'm probably going to have to say Drill Man's stage. I don't think I've ever made it through that stage without the help of Skull Barrier or an E-Tank. Those stupid falling rocks. >_>
Quite franky, the Pharaoh Shot can be used a lot on that stage.
Falling rocks(chunks of dirt?), those flying things that shoot at you, and Lady Bugs, all seem to dive into your sun on purpose.