Tokina AF 16-28mm f/2.8 AT-X Pro SD FX
I don't care much for 3rd party lenses, but for super wide angle in the full-format area Canon doesn't give one much of a choice. The Canon 16-35 isn't worth the money and the 17-40 isn't worth any money at all. So I went for the Tokina.
The Autofocus is - very much contrary to Tokina's claims - neither fast, nore silent. No idea why it's so hard to use USM like Canon does. However, AF isn't a very big issue with super wide angle so I don't care.
Due to the lens's bulb design it's not possible to use filters. I do indeed miss this a lot, but in the end it's the lens's IQ that counts for me. It looks like bulb-design is the key to good IQ in this class. If that means no filters, then so be it.
I ended up selling this lens after some 2 years because it was just too heavy plus I hated that the zoom worked in the other direction than Canon lenses do.