Sigma 150-500mm f5.0-6.3 DG OS HSM
I still do own the Canon EF 70-200mm f/4 USM L IS but I barely ever use it. Almost every time I need a tele (Air Shows, Car Racing, Birds), it's too short, so I got myself this Sigma. I had very strong reservations against 3rd party lenses, especially Sigma, but I had the chance to test it agaist the shorter Canon and, believe me or not - as far as a comparison was possible, due to the focal length difference - I didn't see ANY difference at all. Neither in the center, nore in the borders. I love this lens. Don't miss a faster aperture a lot.
It even works with my Kenko 1.4 converter with some of the contacts disabled, but I have to admit the AF is getting considerably slower and I had an "err" message in the 6D's display every now and then, so it's not a perfect solution.