Not sure that a fast AF is a big deal in landscape shooting, I now use live view for getting focus and composition nailed.
if you like primes, give the 24 TSE lens some consideration. It is pricey and manual focussing, but for landscape, giving focal plane (depth control) and sharpness, it's hard to better.
I use a 17-40 at the moment without complaint, although the corners could be better. Some have said it is sharper and is certainly cheaper, than the 16-35.
I am intrigued with the rumours regarding a Canon 12-24...
Other folks rate the Samyang wides or you could adapt a Nikon 14-24, both of course would be manual.