My favorite is the Tyrannosaurus rex. Not just any T. rex, though--it's Sue. I remember watching her come out of the rock...Then I saw her when they finally got her together at the Field Museum. Damn impressive animal. Strongest bite force of any known. I have a hypothesis regarding their hunting behavior that's one of the more unsettling things I've ever figured out about a fossil critter (and considering my education included a paper entitled "Parasitic Castration in Fossil Crabs", that's saying something!).
I know T. rex wasn't the biggest, meanest, or even scariest-looking, but Sue is the one I grew up with. I have a very personal connection with her. She'll always be my favorite.
My (current) second is Heterodontosaurids. Freaking dinosaurs with horse teeth. I'm not joking--I looked at the photos and thought they were photo-shopped, so I spent my lunch hour tracking down references on the things. They had teeth that functioned exactly like, and looked very similar to, horse teeth. This is as close to a chimera as I've seen in the real world.
I do have to point out that Pterodactyles are disqualified by virtue of not being dinosaurs. Plesiosauria is out as well, for the same reason. Both were Archeasaurs, but neither were dinosaurs.