Wide-angles are excellent for landscapes. These lenses allow to capture the vastness of the landscape, and they give an exaggerated perspective that adds depth to the photos, in particular when you place something in the close foreground. The super wide angles are not "easy" lenses: due to the large angle of view, you have to be very careful about composition.

A wide-angle zoom is the ideal choice: it combines the large angle of view of the wide lens and the flexibility of the zoom. 

All the major camera manufacturer have professional wide-angle zooms in their lineups (e.g. Canon 16-35 f/2.8, Nikkor 17-35 f/2.8). The aperture has not much importance, since you will use small apertures, like f/11 or f/16, to get a large depth of field. The most important things are the range of focal lengths and the sharpness.

Currently, I use the Canon EF-S 10-22 f/3.5-4.5 USM lens ($690), that fits perfectly my EOS 20D, where it gives the equivalent focal lengths of a 16-35mm lens on fullframe. The image quality is excellent, and the built quality is good, even though it is not on par with L lenses. Another good choice is the Sigma 10-20mm - it offers the same image quality of the Canon, but it is much cheaper. The downside of the Sigma is the worse quality control (there are more possibilities to get a bad sample).

I've had for more than one year the Sigma 12-24 EX DG HSM: this is a truly unique lens, with an amazing 122 degrees angle of view on fullframe. I don't recommend it for APS-C cameras - the 10-22 or 10-20 lenses are wider and offers better quality - but if you have a FF camera as the Canon 5D or 1Ds MarkII, the Sigma 12-24 is a dream lens, that offers unparalleled creative possibilities thanks to its ultra-wide angle of view.

A good all-around lens should be versatile: it must have a good range of focal lengths, a relatively wide aperture, excellent image quality, fast AF and preferably image stabilization. There are hundreds of standard zoom lenses from every manufacturer, with prices ranging from $100 for a 18-55 5.6 "kit lens" to $2000 or more for professional zooms.

The Canon EF 24-105mm f/4 L IS USM ($1050) is by far my preferred all around lens: it is very versatile thanks for the wide range of focal lengths and the image stabilization; it bright enough for low-light photography, and it has wheather sealing and a superb built quality. It is the ideal lens for travel and landscapes, in particular on Fullframe (35mm) cameras, but it works really well even with an APS-C camera, if you already have a super-wide angle to cover the shortest focal lengths. A very interesting alternative is the Canon

The Canon EF 100-400mm f/4.5-5.6 L IS USM ($1380) is my preferred tele zoom, even though it is not perfect. It has a very good reach, excellent built quality and fast AF. It has image stabilization, even though is it first generation IS (less effective than the latest version and not useable on tripod). The aperture, while not very bright (f/4-5.6) is an excellent compromise between brightness and portability - a larger aperture would make the lens much bigger and heavier. The image quality is great between 100 and 300mm; at 400mm it is a bit softer, but still quite good, even for large prints.

There are various macro focal lengths, from 50 to 200mm: all them give 1:1 at the min. focussing distance, but the longer lenses have

Since 2005, I use the awesome Sigma 180mm f/3.5 Macro EX ($800), often combined with Sigma 1.4x TC or Sigma 2.0x TC. It is

If you are seriously interested in wildlife and bird photography, you might consider adding a fifth lens to this lineup: the super-tele, that is a 300 f/2.8, a 400 f/2.8, a 500mm f/4 or a 600mm f/4. 

For nearly two years I've had the Canon EF 600 f4 L IS; it is a superb and unique lens, that can be coupled with 1.4x and 2x TC to reach 840 and 1200mm. On the other hand, it is also the biggest, heaviest and most expensive lens of the Canon lineup (it costs nearly $7200). It is a great choice for pro wildlife photographers, even though the 500 f/4 may be a better choice if you have to carry it around a lot (it is smaller and 1.5kg lighter).

Now I've sold the 600 for the Canon 300mm f/2.8 L IS USM (that I'm going to buy very soon) - it has less reach than the 600 f4, but it is a lot smaller and lighter, and it becomes a razor sharp 420mm f/4 with the 1.4x and a very good 600 f/5.6 with the 2x.