Best flash for 70-300 USM IS canon lens?

Your two choices for a dedicated Canon flash are the 430EXII and the 580EXII. For 500mm you will definitely need the 580EXII, but even that might not cut it. I know at least for the 430EX that 200mm is about as long a lens as I will use it with, the subject fitting well inside the frame. Even then it doesnt do much but highlight some stuff. Are you using this for sports? Your best choice might be off camera flash. Or just purchase a lens with an f/2.8 aperture.