0I'd be looking at the sigma 50-500OS, the canon 100-400L and the sigma 120-300.
The sigma has the best focal length range, the 100-400 still has a good range with slightly sharper IQ, faster past ~300mm (the 50-500 is f/6.3 whereas the 100-400 is f/5.6) and it's lighter, the 120-300 f/2.8 is sharper and significantly faster than either but is the most expensive, the heaviest, largest, and lacks IS. You can put a 1.4x on the 120-300 f/2.8 and get a 170-420mm f/4 as well.
If you can do with only 300mm, the canon 70-300L is supposed to be excellent as well, lighter than any of those lenses, very sharp, very fast focussing and weather sealed.
If you can deal with the weight and having to use a mono/tripod most of the time, the 120-300 will outperform any of the other options within that focal length range, but the other lenses offer lighter weight and more flexibility.