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 Ultimate Zoom Lens
The 70-200 f/2.8L II IS Lens is one of the best zoom lenses 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 this focal length range f/2.8 max aperture 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.
Beautifully-Designed Lens that Performs Beautifully
The 70-200mm focal length range works well for a wide variety of subjects and this lens' f/2.8 aperture can stop action in, and blur the background of, many of these venues. The image quality delivered by this lens is stellar and the autofocus performance insures that image quality is realized. This lens looks great and its price is especially attractive.
Impressive Image Quality, Great Build Quality, Fast AF, Excellent Image Stabilization, Compact & Light, Great Value
If you don't need the 70-200 f/2.8L IS II Lens' f/2.8 max aperture, the 70-200 f/4L IS II 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, and you even gain a better IS system than what's found in the f/2.8 model. This is a fantastic landscape and travel lens.
Excellent Image Quality, Pro-Grade Build Quality, Fast and Accurate AF, Very Effective Image Stabilization, Excellent Focal Length Range
Calling this lens a refresh of the wildly-popular original 100-400 L IS I would be a big understatement. The 100-400L IS II is a complete redesign of the earlier lens with many advantages including excellent image quality at all focal length and aperture settings. The 100-400L IS II has the longest focal length available in a Canon zoom lens (without extenders) and is the only native 400mm f/5.6 max aperture lens available with image stabilization. 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, yet the small size afforded by f/5.6 max aperture means that this is a lens that you can carry and hand-hold for long periods of time. This lens is modestly heavy and is not small.
Long Focal Lengths, Great Image Quality, Great-Value Price
Wildlife photographers on a tight budget need to look no further than the Sigma 100-400 Contemporary lens. For a value price, this lens avails the reach-out-and-touch focal lengths so greatly needed for this purpose.
Landscape photographers also want great image quality and long focal lengths are often needed for this pursuit. In this case, great image quality comes in a light and compact (relatively speaking) package that will also be appreciated.
The aperture isn't terribly wide, but this lens is a great value for sports photography, especially for the larger fields.
Great Image Quality, Great Build Quality, Fast AF, Image Stabilization, Relatively Compact & Moderately Light, Great Focal Length Range
The 70-300L 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 70-200L 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.
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 70-200L (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 70-200L 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 70-200L 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.
Big Lens, Crazy Good Focal Length Range, Reasonable Price
For those needing longer focal lengths, but not the short telephoto focal lengths, one of the 150-600mm lenses can be a good choice. I had a hard time deciding which 150-600mm lens I was going to recommend here and there is more than one good choice. The Which 150-600mm Lens Should I Get? page attempts to guide you in that selection process, but I picked the Sigma Contemporary version for this choice due to the low price and good performance.
Visit the Canon Lens Recommendations page for more recommendations.