Student of life in its myriad forms
Other than the flying blue whale…,
…I believe the largest in earth history have been the pterosaurs of the Mesozoic Era, some with wing spans of 9 m (30 ft) and believed capable of flying up to 75 mph.
I think the largest fliers alive today are the Andean and California condors (wing spans up to 3.2 m, 10.6 ft), and a little larger still, the royal albatross with a wing span up to 3.3 m (11 ft).
(1/3 joke answer)
It's basically a tie between Quetzalcoatlus and Hatzegopteryx both with and estimated wing span of about 35 feet. Another pterosaur, Arambourgiania, may have larger but a complete skeleton hasn't been found and estimates range from a 22 to 43 foot wingspan.