A friend of mine does the bulk of his landscape work with a Canon 24-105 F4 (he has just added a 16-35 F4) and, in my opinion his best work, with a Canon 600 F4 L IS. Personally I use the 16-35, 24-70 and 800mm focal lengths for my landscapes (plus, occasionally, 300mm) so without knowing the OP's exact needs it is difficult to make specific recommendations - other than the fact that the long lenses tend to get heavy!
I do really fancy a 24 TSE however!
I also use the 24-70/2.8II and 16-35/4 (because 24mm wasn't enough) for landscapes when depht of view is required to put foreground and background in focus but, when compression is required nothing beats a good telephoto lens.
If you plan to stitch photos simply go to the TSE 24II, which is the best at this FL.