There are two that I really liked: Yrthaks and ravids. Especially the ravids. Those things were awesome.
I do believe Ravids are the only Monster Manual I critters with the official entry Habitat: Positive Energy Plane. I didn't know that until just recently, somehow skipped over it in the past.
I like the mostly unused, often forgotten critters. Which incidentally tend to be the ones the players don't know backwards and forwards. ;)