Characters I have a hard time writing, even though I want to write them

Captain America. The only problem I have with writing Cap is that, well, he's just too sensible to go along with most of my crazy ideas. That's the difference between him and Daredevil. Present Steve with a bad situation, he'll think, "this is going to end badly for all involved", and he won't do it. Present Matt with a bad situation, he'll think, "this is going to end badly for all involved", and then he'll do it anyway.

Deadpool. Few people really write Deadpool well; that's including ficcers and professional writers. You have to find a balance between the tragedy, humanity and insanity of Wade's character, and you have to make him screamingly hilarious at the same time. If I ever get NAILED by an excellent story idea I'll try writing him, but it would be a challenge to get him right.

The Punisher. Frank is a character who thrives in gritty, intricately plotted, over the top, emotionally-exhausting, violent, morally-complex stories AND you have to strike just the right sort of hardboiled urban NYC tone. Very tough to do, I don't think I have the right ear for it, although I have to give Ennis mad props for his Punisher stories.