The questions assumes anything OTHER than a human ofcourse, and limit your answers to real creatures, not fictional or mythological. Feel free to offer your reasoning if you like.
The falcon's eggs are so weak, that the parent's weight sometimes crushes the egg while they are incubating it. (Information from falcon facts) --- I would be a Firefly Squid. Mainly because it lights up, and looks awesome. Image taken off google.
I do really agree with Kalb789! To be the worlds fastest animal; the peregrine falcon. Think the feeling to fly or to swim deep whithout requiring of much air; like a barracuda.