Nearly everyone needs a general purpose lens.
The best Canon general purpose lens - My Canon general purpose lens recommendations ...
Some excellent second or additional general purpose lenses include ...
Canon EF 24mm f/1.4 L II USM Lens
Canon EF 35mm f/1.4 L USM Lens
Canon EF 50mm f/1.4 USM Lens
Canon EF-S 60mm f/2.8 USM Lens (EF-S mount only)
Canon EF 85mm f/1.8 USM Lens (great value)
Canon EF 100mm f/2.0 USM Lens (great value)
Canon EF 100mm f/2.8 L IS USM Macro Lens
Canon EF 100mm f/2.8 USM Macro Lens
And many more ...
Most people are looking for at least one general purpose lens when they buy a Canon DSLR. The heart of the focal length range targeted by Canon for lenses with this purpose is approximately 28-70mm. The DSLR FOVCF (Field of View Crop Factor) comes into play here, effectively requiring a shorter focal length range for 1.3x (APS-H) and 1.6x (APS-C) FOVCF DSLR bodies to yield the same subject framing.
When selecting a lens, I generally look for the highest image quality I can get in the most versatile and highest quality package I can get it in. I always want great autofocus performance as well. My personal highest recommendations for general purpose lenses are the Canon EF-S 17-55mm f/2.8 IS USM Lens, Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens, Canon EF 24-105mm f/4 L USM Lens and Canon EF 24-70mm f/2.8 L II USM Lens.
Prior to the introduction of the EF-S 17-55, EF-S 15-85 and EF 24-105 L, the Canon EF 24-70mm f/2.8 L USM Lens was what I considered the ultimate general purpose lens. The replacement for this lens, the Canon EF 24-70mm f/2.8 L II USM Lens, offers considerably better image quality and faster AF. I use the Canon EF 70-200mm f/2.8 L IS II USM Lens with more frequency on a full frame body, but still recommend the 24-70 as a one-lens-does-everything selection.
The 24-70's 24mm end is wide enough for environmental portraits and wide landscapes while 70mm makes a reasonable head/shoulders portrait focal length. Group pictures, birthday parties, weddings, photojournalism, travel pictures ... the uses are endless. The image quality from this lens is unsurpassed by any Canon EF lens containing this focal length range with the 24-105 L (next) being the only possible exception. And with a fixed f/2.8 aperture, this lens is as fast as any zoom lens. Build quality is excellent.
The Canon EF 24-105mm f/4 L IS USM Lens joined my most-recommended list after the 24-70 L (it was released after the 24-70 L). The Image Stabilization and additional focal length on the 24-105 offset (and for many situations surpass) the benefits of the fixed f/2.8 aperture on the 24-70 L (stopping action in low light is not one of them). Especially when shooting with a full-frame body, I find the extra 35mm of focal length on the long end very valuable in a general purpose lens - and especially for portraits. Image, build and mechanical qualities of this lens are top of the line. The 24-105 L's size and weight make it a very nice handling lens.
The Canon EF-S 17-55mm f/2.8 IS USM Lens and Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens were introduced after both of the aforementioned L lenses and I now highly recommend them to APS-C/1.6x body users.
When using a 1.6x FOVCF body, we often want wider focal lengths from our general purpose Canon lenses. The Canon EF-S 17-55mm f/2.8 IS USM Lens (EF-S mount only) delivers excellent image quality, a wide aperture and an excellent implementation of image stabilization. Fast, quiet and accurate Ring USM AF is a valuable asset of this lens.
The Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens does not have a max aperture as wide as the EF-S 17-55, but it offers a wider focal length range in a smaller/lighter package that costs even less. It is a great lens if you do not need to stop action in low light - one of the primary advantages of an f/2.8 max aperture lens. The EF-S 15-85 also has IS and Ring USM AF. It delivers sharp images.
Not everyone wants to spend as much as the above lenses cost. For these people, I recommend the Tamron 17-50mm f/2.8 XR Di II Lens (EF-S mount only) and Canon EF 17-40mm f/4.0 L USM Lens. Both lenses are very good values with excellent image quality and relatively small size/light weight.
A very good value Canon general purpose lens choice is the Canon EF 28-135mm f/3.5-5.6 IS USM Lens. This lens is not very wide - especially on an APS-C/1.6x body, but the telephoto lens is really nice to have. This lens has both Ring USM AF and Image Stabilization.
Pair a general purpose zoom with one of Canon 70-200mm L lenses for a great kit - the Canon EF 70-200mm f/4 L USM Lens is a great value.
Read the full reviews to see which lens(es) are best for you.