This may be a little surprising, but: most mice, though Rizzo's the worst culprit for me. I kind of like AC mice, but they bring back bad memories for me. My main gripe with Rizzo, though, is the fact that I can never figure out his little headscarf thing. It kind of creeps me out, admittedly, just with how it's basically tied onto his nose... His eyes, too, only make it worse.
And don't get me started on the way Samson smiles. He scares me with that expression.