Well, from Canon, there are many excellent lenses for landscape: the TSE series (especially the 17mm and 24mm); the new 24-70L f/2.8 MKII zoom (which is excepcionally sharp); the 70-200L f/2.8 MKII zoom; the 100L f/2.8 macro... the zooms are really very good, equivalent to primes, and offer very useful flexibility for landscape shooting.
I would add the TSE 90mm. Exceptionally sharp and good lens inside the whole image circle, making it ideal for stitching. The image attached is taken with a 5D III and the TSE 90 and is a stitch of three images using the shift of the lens (and some tilt). Only takes me 15 seconds to take and gives me an 40+ mpixels image.