My favorite is the secretarybird. They're gorgeous, tall, they can fly, they kick snakes to death and then eat them, what's not to love?

My favorite extinct dinosaur is the microraptor, a relative of deinonychus that had four (!) wings. Two pairs, one on its forelimbs like modern dinosaurs and one on its hindlimbs. Most people draw them like four-winged crows with teeth, which is great because crows are also crazy smart and probably some of the smartest animals besides humans and maybe chimps. They know how to use cars to break open nuts by dropping them onto an intersection and they wait for the lights to change before they fly back down to collect the spoils. I'm typing so fast that autocorrect almost can't keep up now so I'll stop for now

Edit: apparently autocorrect wasn't quite keeping up