I'm very pleased with my 17-40, actually - one of my favorite lenses. I use it exclusively on my 5D Mk II and I'm very happy with the results. I also love the portability of the 17-40 which makes it great for travel. I'm not a landscape-shooter per-se but I do extensively use this lens when traveling.
Supposedly it's the best WA zoom Canon has to offer, so I would suggest you look into a prime as suggested by others on this forum (though a T&S would perhaps be a little overkill). How about a 24mm L? Note I've no experience with the latter. I do have a Samyang 14mm and a Sigma 20mm - interesting lenses but not for landscape work I'd say...
« Last Edit: July 14, 2013, 11:58:26 AM by mrsfotografie »
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses