Dinosaurs are pretty spectacular, but truth be told, I find pterosaurs a lot more interesting. They first appeared in the fossil record around the same time as mammals and dinosaurs, and like dinosaurs, quickly evolved to fill an absurdly wide range of niches.
While I tend to stay away from any overly fantastic extrapolations of the fossil record (à la Jurassic Park), I can't help but side with the liberal analyses of pterosaur anatomy and behavior by David Unwin and like. While a great deal of my bias likely stems from my hatred of birds, it has to be ceded that any airborne creature as large as some of the late Cretaceous pterosaurs would certainly have been a model of efficiency.
I mean, seriously, imagine a creature with a wingspan of 10m or more weighing less than an average human.
That's fucking impressive.