Begins and ends with Mysterio if we're talking "high flying...ness". I think Eddie and Steamboat are better than him at working, but yeah if this is a question of who uses aerial stuff in WWE best than ReyRey's the man.
That said, Hardy is willing to put himself in the most dangerous situations to pull off a high spot, so I'd give it to him, only because he is more daring than any other wrestler I've seen. The guy just doesnt seem to care what happens when he's out there. Mysterio doesnt have that, and thats probably a good thing.