Since everyone else chose eggs, I guess I'll pick something different. ;)
I'd have to say coconut, in its many forms. I cook my foods in coconut oil, bind things like meatloaves with coconut flour (one tablespoon goes a long way!), and eat coconut butter by the spoonful for dessert. If there were any coconut milk in the house, I'd readily consume that, too.
Why? Availability, food sensitives (afraid to try butter due to acne during my last attempt, hard to find ghee and not ready to commit to a bulk online order when I know coconut oil works well for me), and palate. I love the taste of coconut, and I love the versatility of it as well. Plus, where I live, it's readily accessed at the Co-op or natural food store. :)