After four years with a Canon 1Ds then a 1Ds MkII, I've found two really good Canon wide angles, the 35mm 1.4L and the 14mm L. These two can stand comparison with any retrofocus lens that I've ever come across.
After this the field thins out quickly. I used the Canon 20mm for a while but eventually sold it. It was okay, certainly not a dog, but that's all it was.
I use the 24mm T&S regularly and I've always found it to be a very solid performer, not in quite the same league as say a Leica M 24mm apo but still very respectable. Recently I evaluated the 24mm T&S, the 24-105 4.0L IS, and the Zeiss Distagon 25mm used with a Zeiss/Canon adaptor. In terms of sharpness the 24-105 was fractionally ahead of the T&S which was fractionally ahead of the Zeiss Distagon. The zoom really is sharp, but sufffers from quite bad distortion. It may not be an issue for you, but it's something I dislike. The DxO RAW converter can correct the 24-105's distortion convincingly, but that either means using DxO for all RAW conversion, or adding an extra step to the conversion process (now that DxO supports DNG).
What I'm hoping for is the Canon equal of the Zeiss 21mm, biting corner-to-corner sharpness, great flare control, and reasonable distortion.
There was an interview about six months ago with a senior Canon executive who admitted that Canon sensor development had gotten ahead of lens development, particularly wide angles. Given that Japanese managers rarely publicly recognise deficiencies unless they've got a solution in mind, I've got my fingers crossed that something impressive with a red ring in the 18-24mm range isn't too far away!