It was the promise of the lenses that made me switch from Leica & Nikon over to Canon. I don’t think that promise has been broken. All have been razor sharp to my eye except one. Their lenses render colors bright and clear. Over the years I have been able to expand my range of lenses with forward compatibility. Obsolete lenses of the 90s still work fine on the cameras of 2011.
The 5D Mark II SLR camera, my current choice, comes as a body or in a kit with a 24-105mm f4 lens. I bought it as a kit. There are other lenses of course. My nephew happened on a 24-70mm f2.8 Canon lens. He swears by it. I like the 24-105 because it covers 90% of the shooting situations I have always found myself in. Using a filter the lens will vignette slightly at 24mm. Fortunately the effect is quite small and is usually cropped out with little or no distress to the composition.
I have found the 24-105 to be so broad in its usefulness that I have not used my 16-35mm f2.8 nearly as much as I originally anticipated.
Both Nikon and Canon cover a broad range of focal lengths. They have fisheyes and they both seem to start at about 14 mm super wides, which gives about a 114 degree field of view. Canon now makes an 8-15mm f4 fisheye zoom that moves from a round picture to a full rectilinear frame. In the 70s & 80s Nikon even made a 6mm f/2.8 ultra, ultra wide lens. It was a very large manual focus lens with at least a 220 degree view!
Both brands cover all sorts of prime focal lengths and zooms. Both brands top out with 80-400mm or 100-400mm variable aperture zoom lenses. I find the Canon 100-400 to be versatile & sharp. It can be hand held at very fast shutter speeds with good success.
In the super telephoto area Nikon seems to top out at 600mm while Canon reaches 800mm. Years ago Canon even made a 1200mm f/5.6 (non reflector) super telephoto on special order. Both brands make 300mm f4 lenses in a reasonable price range. Coupled with an extender these lenses offer a pretty good reach for around $2,000.
Both companies also offer a range of specialized lenses for specific types of work. There are macros & tilt shift lenses to choose from.
In the past the widest I had ever gone was 19mm. The longest was 300mm with a 2x extender. I never liked the way the extender treated the photo so I shelved it and traded it in on my digital kit later.
When I traded in my old film cameras and lenses for digital I kept my 300mm f4 telephoto. I traded for a 16-35mm f2.8 zoom, the 24-105mm mentioned above and a 100-400mm variable aperture zoom. I added a 15mm f2.8 fisheye a month later. All that time I thought the 100-400 was a large lens, until in a moment of weakness I bought a 70-200mm f2.8 zoom. Now that is a bazooka! I should have gotten an f4 because it is smaller and lighter for field work.
Of all these lenses I use the 24-105mm most, but I think the fisheye offers the most fun. You will see this here and there as I show photos from the fisheye on the website.