Well, from what I've been doing so far. 7-7 and 7-18 are a pain in the ass.
7-7 because I have an (ir)rational hatred of fans.
7-18 because it's just plain evil.
If you're going for the hardest stages first, then I'd definitely put those two on the top of the list.
When I read that 7-18 is just plain evil, I thought that it's not so hard...and now I have a strange feeling that I'm gonna regret that thought
One of my hardest levels are 7-4, 7-7 and 7-10.