Rock n' Roll Racing strangely hasn't been mentioned yet.
I dunno, there are so many good SNES games it's kind of hard to keep track and I'm not next to my collection.
Illusion of Gaia isn't tough but it isn't very good either
edit: oh and I should mention The Twisted Tales of Spike McFang since I doubt anyone else will...
I'm also a sucker for Paladin's Quest but everyone else tells me it sucks... oh well.
another edit: Oh yeah and if you are emulating, pick up a translation and rom of Treasures of the Rudras. Good shiz.
I'll think of more when I'm not at work...