This is such a broad term. I think in terms of flying as those who use flying as there primary means of getting around. I would exclude all high cosmics like Galactus and other high beings who can fly if they chose to do so and could increase their flight speed as much as they would like. You also have fliers who can travel fast but not in earth's atmosphere do to certain limitations. For example Surfer can travel at light speed but he won't in earth's atmosphere. While Photon can travel light speed in earth's atmosphere without even ill effects. I willl limit my repsonse to fliers that can travel in space with out limitations.
1.) Silver Surfer
3.) Thor and Beta Ray Bill
5.) Photon (she is maxed at the speed of light)
There may be more but I can't think of them at the moment.