For most photographers, a telephoto zoom lens is going to be the second-most important lens in their kit. This is the lens that is going to be pulled out for portraits, events (including kid performances), sports, wildlife, landscapes, the beach and many more photography subjects.
The approximately 70-200mm focal length range is what I consider the base need for this lens. APS-C/1.6x format cameras will have greater need for shorter focal lengths and full frame format cameras have greater need for longer focal lengths in specific instances, but this approximate range works well for both camera formats. And this range aligns nicely with most general purpose lens focal length ranges.
Note: Don't worry about an overlap or a small gap between the focal length ranges of your lenses. The overlap is a positive attribute - and can mean fewer lens changes for you. A small gap (say 55-70mm or 70-100mm), while not a positive attribute, is usually not a big issue for most. Selecting the right lenses for each use should be a higher priority.
The following examples were captured with a full frame camera.
The Best Canon General Purpose Telephoto Zoom Lens - My Recommendations
The Ultimate Zoom Lens
The 70-200 f/2.8 L II IS Lens is the best zoom lens I've ever used. This completely professional grade lens has incredible image quality, fast and accurate AF and a late-generation image stabilization system in a solid, weather-sealed, fixed-size body. The wide-as-it-gets-in-a-zoom-lens f/2.8 max aperture over the entire focal length range allows action to be captured in low light and allows the background to be blurred away. This lens performs very well with extenders - for when you need even more focal length. This is one of my most-used lenses - I would be lost without it. Price and weight are this lens' two downsides.
Impressive Image Quality, Great Build Quality, Fast AF, Image Stabilization, Compact & Light, Great Value
If you don't need the 70-200 f/2.8 L II IS Lens' f/2.8 max aperture, the 70-200 f/4 L IS will give you everything else in a smaller, lighter and less expensive package. Without the f/2.8 aperture, you will need twice as much light to stop subject motion (at f/4) - and you will not be able to create a background as diffusely blurred. But, you also give up a substantial amount of size (still fixed), weight and cost. You give up very little in image quality or other pro-grade features including solid, weather-sealed build quality. This is a fantastic landscape and travel lens.
Great Image Quality, Great Build Quality, Fast AF, Image Stabilization, Relatively Compact & Moderately Light, Great Focal Length Range
The 70-300 L IS has become my most-used landscape and travel telephoto lens. This lens does not have an especially wide (or fixed) max aperture and it extends a noticeable amount by 300mm, but it has a longer focal length range than the 70-200 L lenses. And very nice image quality. Excellent build quality including weather sealing is also a feature of this lens. Exactly what you would expect in a red-ringed (L-series) Canon lens.
Very Good Image Quality, Great Build Quality, Fast AF, Image Stabilization, Excellent Focal Length Range
The 100-400 L IS is showing some age and the push-pull zoom mechanism is not preferred by all, but it is hard to argue with that 400mm image stabilized max focal length. When wildlife, surfers, aircraft, large-field athletes and other small/distant subjects are in the viewfinder, you will greatly appreciate having 400mm to call upon. This lens is modestly heavy and is not small.
Excellent Value, Very Good Image Quality, Pro-Grade Build Quality, Fast AF
While the Canon EF 70-300mm f/4-5.6 IS USM Lens (the non-L version) is a more popular lens, the 70-200 L (any of the 4 current models) is a much better lens from image quality, AF performance and build quality perspectives. The 70-300 IS has image stabilization, a longer focal length and a lower price to its advantage, but I highly recommend photographers of all levels take this step into the Canon L lineup. The 70-200 L f/4 non-IS is a great value and a very strong performer. I don't remember anyone ever telling me that they regretted this decision - but I have heard from many happy 70-200 L owners.
Impressive Image Quality, Wide Apertures, Relatively Small Size/Light Weight
Yes, this page is primarily about helping you choose the best telephoto zoom lens for your needs, but ... if a fixed focal length (prime) telephoto lens will work for you, Canon has a huge range of superb choices. It is hard to go wrong with any L-series telephoto lens model.
Very good image quality for a bargain price
If your budget is tight, this might be the right lens for you. The 55-250mm focal length is very nice and the image quality per dollar ratio is high. What is not high is the build quality of this lens which includes a plastic lens mount.
Visit the Canon Lenses page for more recommendations.