The Canon EF 70-200mm f/2.8L IS II USM is one of Canon's best lenses, and unlike the shorter 24-70mm, includes image stabilization. The lens is well-rated in image testing, especially for a lens with such a wide zoom range. Images are sharp and distortion and chromatic aberration is minimal. With a fast f/2.8 aperture throughout the zoom range, it's easy to see why this lens is the choice of many high-demand professionals.
Of course, packing the sharpness, zoom range and wide aperture (not to mention weather sealing) into one lens has its downsides. Mainly, this thing is huge, over three pounds and will certainly add some weight to your bag. It's tough to shoot with for long periods without a tripod or monopod to take some of the weight off your shoulders. All the features also make it one of Canon's pricier lenses at around two grand. But if you need telephoto versatility with high end image quality, this is the lens to get.