For the money, the Tamron SP 70-200mm F/2.8 Di VC USD G2 is a really solid telezoom. It's a tough lens that can be used in any sort of weather, and delivers images with superb sharpness through most of its range.
Excellent resolution through most of range; Optical stabilization system; Bright f/2.8 aperture; Sturdy build quality; Teleconverter compatibility; Integrated tripod collar; Available for Canon and Nikon cameras; Affordable
Weak flare resistance; Dimmed corners; Sharpness drops off at 200mm; Toggle switches prone to inadvertent changes