It used to be flying very low, or the moment when you punch up through a layer of cloud, but to be honest my tastes have changed with more flying experience and now its just about getting places faster than I can in a car.
The greatest thrill for me would have to be jumping out! Specifically leaping from the ramp of a C-130 and going through the drills as the rush of accelleration and noise hits you! It's better off the ramp than the side door because you get the "big picture" effect just before you jump. Love it.