What amazes me is the human eye-brain. I don't think of the eyeball as a separate organ from the brain, because the eye is essentially an evolved nerve—and it's nothing without the brain. The same argument can be made in reverse, when you consider how much information comes into the brain from the eye. The only comparable system is in some of the cephalopods, specifically octopuses and squids. While some animals have specific visual abilities that far exceed ours, no others have a system as generally capable as ours, with the possible exception of the two I mentioned.