It has been said that crows are the enemies of the hawk. They have learned that 'mobbing' (the correct term) distracts the hawk so it can't hunt and generally so badly annoys the bird that it leaves the area. With a hawk gone, the crows are safer so they're really doing a 'neighborhood watch' which includes driving the 'undesirables' out of the neighborhood.
Hawks' exhibit traits, a few of which are similar to humans. The female sends the male out to get food for the family while she sits with the hawklings, spies on the neighbors and nibbles all day on leftovers from the nite before . Eventually, The male comes home after a hard day of preying and is greeted with nothing but a cackling wife and kids, they don't say hello but just rip the food from his person. After they eat the kids go to sleep and the female starts yelling at him moaning about what a hard day she's had, and why he can't even help clean the nest by using the pooper scooper and picking up some stray feathers. He gets fed up, snaps at her then goes to sleep, So in answer to the question, the hawks worst enemy is the female of their species.
Mostly gamekeepers, as hawks kill young game birds.