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I suppose my current favorite is the 24/3.5 TS-E II, but unless I'm shooting for a particular subject and just one approach to it, I always want to have at least two lenses, and preferably three with me. For example, 16-35, 24 TS-E, 70-200/4. Or 17 TS-E, 24-105, 100-400. I know Dan Mitchell has lots of examples shot with a 70-200, and I recall a few with 100-400. I sometimes find myself making more shots with one of those than with an ultra wide, depending upon the details and shape of the landscape.
A good lightweight combo with your 17-40 would be to pair it with a 70-200/4. Wider shots can be done by stitching. Playing with a zoom into the telephoto range can bring out interesting ways of seeing. If you get the bug for a tilt-shift and can budget for it, I'd suggest starting with the 24.