Having that said, I think most people agree that the majority of the Canon L "pro" glass are superior in most ways to the non-L "consumer" lenses. They are usually optically sharper, faster, and will open more photography-opportunity doors for you. Some people believe that looking at MTF curves on the Photodo website is all that matters, but honestly, who gives a sh!t about curve fall-off when the lens did not make the shot.
I've found there are some exceptions to this "L-is-better" rule:
1) The 16/17-35 2.8L is not very sharp, but is one of the most-frequently used lenses in the Canon arsenal. It's extremely versatile, is rectilinear(!), and is relatively affordable for what it is capable of.
2) The 100/2.8 Macro is NOT an L, but is as sharp, if not sharper, than some of the prime Ls. Its USM is just as fast.
3) The 85 1.2L is optically fast, but don't bank on its autofocus being able to capture a hummingbird flying around. The front element is so heavy that the autofocus is retardedly slow.
4) The 50/1.4-1.8 does not seem to be any less sharp than the 50/1.0. Sure, it is generally known that a stop or two down from the max aperture is the sharpest, but cmon, you really have to try it out to see. Plus the 50/1.4 is $300, and the 50/1.0 is $2000.
5) I find the 70-200 4.0L is as sharp as the 70-200 2.8L. It is half the cost.
6) The 28-135 IS definitely is one of the most versatile lenses, and is pretty sharp. Is it 100/2.8 sharp? No. Is it optically fast? No, it is 3.5-5.6. But it is very affordable (~$500), has IS, and is good enough for most quick photography that does not permit changing lenses.
7) I would tend to just forget about those <$150 lenses, like the 20-90mm or 20-80mm freebie lenses that they give away with the camera bodies, WITH THE EXCEPTION of the 50/1.8. What a great lens!
See what I mean? I find it really hard to place a real quality rank ladder; it would probably be a much easier question if there are specific photographic needs that you want to fill.
Sorry for the rambling,
Keith
