This question is another one that is probably more meaningful to 3e/4e players than old school D&D pleyers. I’m not sure if the question means “which animal do you like best as a monster” or “which animalish monster” — I mean, is a chimera an animal? All the parts are basically animal… by that standard I probably like owl bears the most. I know they are kind of a stupid idea — what exactly about an owl makes a bear more dangerous or scary, really? Especially a non-flying owl.
On the other hand they do look awesome and the beak may be an improvement over teeth. Especially the toothy beak you see in some illustrations.
Dave Sutherland’s owlbear is kind of a sad sack, at least compared to the comic-booky illustrations you see these days, but there is also something really creepy about those almond-shaped eyes (more human than owl) and the hunched back.
As far as “vermin” go, I can’t say I have a favorite. I think the choices are probably giant rats, giant centipedes, and bats? Those are all good.