I love Snape; I think he's such an awful teacher, but there's something about him that just makes me laugh.
He gets some of the best lines, and I love the desrciptions of him; the way he 'glides' everywhere and has those fantastic black robes. I know he's not strictly evil, but the bad guys are always the best.
But honestly, if I'd ever had a teacher like him, he'd have been reported straight away! *laughs* (although I think the worst teacher would have to be Umbridge, hem hem ;D)