I think the first question is to ask yourself, how often do you really need to go wider than 24mm, then consider hard your answer and ask yourself why. While soms landscapes do work well at wideangle, not as many do as a lot of people assume, plus filter use starts to become a problem, unless you have the larger Lee filters or equivalent - not cheap. Personally, I rarely find a need to go wider than 24mm on full frame, but everyone is different. If after your strong consideration, you definitely want to go for a wider view, then aside from the lenses already suggested, consider the Zeiss 18mm and 21mm Distagon lenses (I know they were mentioned in passing). The 21mm is supposed to be the better of the two, but the 18mm does well in tests (certainly better then the 16-35L MkII) and is wider. I would certainly rule out the 17-40, as mentioned, it is very soft in the corners at 17-20mm and if you take any pride in your landscape work, you won't be happy. That said, it is probably on a par or slightly better than the 24-105 once you get to 24mm and beyond.