You'll need something at the wider end of normal, and presuming you're shooting landscapes a zoom is more useful than a fixed focal length lens.
Just my take, there are 2 Canon lenses worth considering.
Canon's 24-70 f:2.8 is heavy like a brick, it does not provide image stabilization, and you'll constantly find yourself wishing for a few more millimeters at the long end. On the other hand it has very little vignetting if any, it is extremely sharp at any aperture from f:4 down, and the lens hood is actually useful because the lens barrel extends as you zoom wider, so coverage of the hood matches the focal length pretty well.
Canon's 24-105 f:4 is light enough to be handy, long enough to be useful for portaits in a pinch, and it's image stablized. On the other hand...it vignettes badly, even if you stop down, and corners tend to go a bit soft at any but optimal (f:8 to f:11) apertures. The lens hood is worthless because it's so shallow it never really protects the front element from rain or stray light. And no matter how carefully you use it, the images won't be quite as sharp as what you could have gotten with the 24-70.
I own both lenses. I routinely take the 24-70 and a 70-200 f:2.8 mounted on two bodies when I'm out shooting landscapes, and I'm generally very happy with the results. The 24-105 always seemed to deliver results that are...near great rather than great. I do however use the 24-105 all the time as my solo lens when I'm walking around with one body or shooting people pictures where more sharpness is not necessarily a virtue.
Don't bother looking at the 16-35; I've owned both versions, and they just don't measure up in terms of corner sharpness at any aperture. I much prefer using a better lens and stitching panos rather than wasting that good sensor with a mediocre lens.
