I'm principally a nature photographer shooting landscapes with wide angle lenses, so that influences my choice of lenses. Most of the time I'm looking for depth of field, so I'm working stopped down, typically f8 to f16. This means that I really don't need fast lenses, such as those a photojournalist might use. This is something of an advatage, since I don't need the relatively large, fast, heavy and expensive zooms like the 17-35/2.8 and 28-70/2.8. These fast lenses are clearly superior for low light work where DOF isn't an issue, but when working stopped down they really aren't that much better than the smaller, lighter, cheaper "consumer zooms". For most people the slightly better optics aren't worth the significantly (3 to 5 times) higher cost. 

I've used the 20-35/3.5-4.5 USM, the 24/2.8, 28-105/3.5-4.5 and 28-135/3.5-5.6IS so I'll confine my direct comments to these lenses. I'll make brief comments on the 20/2.8 and 24-85/3.5-4.5, but since I haven't used them myself, I'll not say too much about their performance. 
 

Canon EF28-105/3.5-4.5 USM 

This is a great lens. It's small, light and inexpensive. Sharpness is excellent and it covers a very useful 28-105mm range. 28mm is the longest "true" wideangle and this lens capable of excellent wide angle landscape work. As I've said, I typically shoot landscapes at f8-f16 and this lens shows no vignetting, even with a standard tiffen circular polarizer. It takes 58mm filters, a common size in the Canon lineup. There's a full review of this lens on photo.net. If your finances are limited and you can only afford one lens, this (or maybe the 24-85/3.5-4.5) is probably the one to get. The newer 28-135 IS is also a contender now. 
 

Canon EF28-135/3.5-5.6 IS USM

The 28-135/3.5-5.6 IS USM is significantly larger than the 28-105/3/5-4.5 and takes larger filters (72mm vs 58mm). It's also heavier and more expensive. So is it worth the added size, weight and cost? My answer is that it is, for two reasons. First there is the built in IS (Image Stabilization), which allows you to hand hold the lens at shutter speeds 2 or 3 stops slower than you would be able to without IS and still get sharp images. Second, in my testing I found that the optical perfomance of the 28-135 was slightly better than that of the 28-105. The 28-105 isn't bad, but the 28-135 is better. If you want a smaller, lighter, cheaper lens, the 28-105 is a good way to go, but if you want a lens you can hand hold in lower light, plus a slight edge in optics, the 28-135 is the better choice.

Canon 24/2.8 

The 24/2.8 isn't a USM lens. It uses a micromotor which means you can hear it focus and you don't have full time manual focus. I really don't mind either on a wide angle lens. The advantage of the prime over the zooms is that it's smaller and lighter. The speed really isn't much of a benefit for landscape work though. What it does have is a floating element design, which means that the optical formula of the lens changes when you close focus to maintain optimum sharpness by minimizing spherical aberration and field curvature. This can be an advantage if you do a lot of closeup work. In that case, the fact that the 24 prime focuses down to just under 10", while the 20-35/3.5-4.5 only focus to 13" could also be a point to consider. 3" doesn't sound like a lot, but for a wide angle used that close, it's significant. The other major advantage of the prime is lower flare. I've compared shots taken of the same scene with the 24/2.8, 20-35/3.5-4.5 and 28-105/3.5-4.5. With the sun in the frame, both zooms do a creditable job of flare supression, but nevertheless flare is evident. The 24 prime is significantly better, though some flare can still be seen under such adverse conditions. Perhaps surprisingly, the sharpness of the zooms is comparable to that of the prime when working at f8-f16, so sharpness isn't really a big factor when deciding between these zooms and the prime. The 24/2.8 uses a 58mm filter, which can be very convenient since that's the same size as many other Canon lenses such as the 28-105/3.5-4.5, the 75-300/4-5.6 and the 100-300/4.5-5.6. 58mm filters are often relatively inexpensive too.The 24/2.8 also has a traditional depth of field scale on the lens, which can be useful. 
 

Canon 20-35/3.5-4.5 USM 

Another USM lens with silent operation and full time manual focus. Well constructed and a good performer, this might just be the "best buy" in a Canon wideangle. Sharpness is good, especially stopped down, flare control is good (but not as good as the 24/2.8 prime) and vignetting is negligable, especially when stopped down. I have no problems using a standard Tiffen 77mm polarizer with this lens. 77mm is a "standard" Canon filter size and is used on lenses like the 70-210/2.8 and 300/4, so you might already have filters which fit this lens. If you have to buy filters, some, like enhancing filters, can be pretty expensive in a 77mm size,so bear this in mind. 
 

Other Canon Wide Angles 

The obvious other choices are the 20/2.8 , the 28-135/3.5-5.6 IS and 24-85/3.5-4.5. The only downside of a 20mm lens is that it can be difficult to use. 20mm is pretty wide and you really need to like the "20mm look" to have a lens dedicated to that focal length. The 24-85/3.5-4.5 covers a nice range. 24mm is a good focal length. In fact if you could chose only one wide angle focal length, 24mm would probably be it. The major downside of this lens is the odd 67mm filter size, used by no other Canon lens. Since filters (split ND, polarizers, warming filters, enhancers) are often used for wide-angle landscapes, this can be an issue. You either have to buy (and carry) a new set of 67mm filters, or use a step down ring and larger filters, which means you can't use the lens hood. The 24-85/3.5-4.5 is reviewed on photo.net.

What to buy? 

This is a tough question because it depends on what other lenses you have. If you have a 35-whatever zoom, both the 24/2.8 and 20-35/3.5-4.5 make sense. Even the inexpensive 28/2.8 could be a good choice. If you have a 28-whatever zoom, the difference between a 28 and 24mm is pretty small and it might be better to go with the 20-35/3/5-4.5 zoom, or possibly a 20mm prime. If you have a 24-85/3.5-4.5, optically, only the 20/2.8 would really make sense, though it's actually more expensive than the 20-35/3.5-4.5 zoom, so the zoom's still a reasonable option, at least from an economic viewpoint! From a sharpness point of view, for landscape work where you are at f8-f16 I don't think the extra cost of the fast 2.8 zooms is cost-effective, and even any sharpness advantages of the primes over the slower zooms is pretty small. I would chose a lens that covered the focal length I wanted and had the features I wanted (filter size, weight etc.), with optical differences being a smaller weighting factor. If you intend to shoot a lot under flare provoking conditions though (e.g. with the sun in the frame), that's the time to give extra consideration to the prime lenses. Primes also typically show less distortion, though again, for landscape work, this isn't usually a big issue 

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