I think their tongues are the most amazing adaptation. I remember learning about this in a lecture from a ranger in Yosemite National Park years ago. It was the beginning of my fascination with these remarkable birds.
Diversity, and size variation. Also the mimicry between genetically unrelated species was really interesting living in an area where that occurs in Northern California. Also I'm a big fan of acorn woodpeckers so I enjoy the explanatory video.