Sigma 35mm 1.4 ART
I have used the canon 35mm 1.4L in the past, and that is a great lens. The colors were great, the focus is quick, and it's very sharp. However, the sigma 35mm 1.4 art is just plain better (in most ways).
The sigma is SHARP as heck wide open. If you shoot 35mm lenses on full frame cameras, you can pretty much shoot it wide open all day long because the depth of field is great enough to allow for that in most situations. I almost never stop down my sigma 35 and it has amazing colors, contrast, and sharpness. The build quality of the art line is exceptional as well. One drawback of this lens is that it heavily vingettes wide open, which to me is not a problem most of the time, but some times you may think you nailed an exposure only to find out that all your highlights in the center of the frame are blown out.
If you don't have this lens in your bag, get it. I was not crazy about the 35mm focal length at first, because it can make people ugly and is certainly not a portrait lens, but then I realized that about half or more of my wedding photos are taken with this lens. It allows you to get a good amount in your frame at a wide aperture, and it's certainly a story telling lens. I use this lens for MOST of my exit shots, getting ready photos, and detail photos. If you are debating between this lens and the canon L, save the money and get the better lens.
I have noticed that sigma's calibration quality control is a bit lacking however, so you are going to want to buy the sigma dock along with any of the art lenses. Fortunately the dock lets you calibrate the lens at various distances and fix sigma's lazy/sloppy calibration from factory. My first copy of this lens was PERFECT, but now that I have switched to nikon and purchased one in that mount, I've had to do all sorts of adjustments. Still worth it, though.