I wonder what other lenses you already have? My leisure photography (my work is mostly product) takes in natural as well as urban landscapes and building interiors. I agree with Rambarra in that a 24mm focal length is probably the most useful general purpose natural landscape focal length. There seems to be this, 'wider is better' train of thought for landscapes, but in my (and obviously Rambarra's) experience, it takes significantly more knowledge, experience and time to make a good image with a wider lens. Simply cramming in more stuff doesn't make for a great image - composition is everything. I don't suggest you shouldn't go ultra-wide, but you should learn your stuff on a 24mm first. Certainly no wider than 21 initially.
I think everyone should start with a good general purpose lens, be it a zoom or say, a 35mm or 50mm lens. Realistically, the 24-70 range is what most of the people use, most of the time, and so if you don't already have decent lenses in this range, I'd start with the 24-70 II. With a 5D3, the 24-70 II is superb, although it's not quite as sharp as its reputation would suggest at short distances. Beyond 15 feet or so, it's superb in every respect. The weight of the body/lens combo goes a long way to give you greater stability, especially at the wider end. You really will not miss IS unless your physical technique is shocking. Elbows in, and all that. Having said that, if you're seriously considering blowing this kind of money on equipment for landscapes, the first thing you should be planning for is a top class lightweight tripod, which of course renders IS irrelevant. AF is also irrelevant as you should be zone-focusing (using hyperfocal distance), which also gets around the fact that the focusing screen on the 5D3 is not interchangeable for manual focus lenses (my main bugbear with the 5D3). I'd been using the 21mm Zeiss for a long time before I got a 24-70 II, and I will admit that, generally, the Zeiss doesn't get quite so much use as it used to (when my other options were 35mm and 50mm). But...BUT...21mm is significantly wider than 24mm on full frame and these two focal lengths do not render each other obsolete. By no means. As others have said, the Zeiss is a superbly made bit of kit, and certainly a better technical solution to landscapes than the 24-70 if the focal length works for you. I've also recently found myself dabbling with starscapes, i.e. pictures of the night sky, and at this, the Zeiss excels. At f/2.8 it's beautifully sharp right to the edges, without significant coma, and the extra 3mm of focal length makes a huge difference in this use.
Then, there's the TS-E 24mm. This is a much harder lens to master, but if you put the time into it, it will repay your efforts handsomely. Tripod is obligatory, here. I'd say it's the sharpest of the three, but not by a huge margin (not really enough to sway your choice). Of course, you can stitch panoramas together simply by shifting fully left then fully right without moving the camera, which gives you a much wider effective focal length, down to 17-ish. However, if I was only looking for two lenses, I think I'd find it hard to justify the TS-E 24 as well as the 24-70. The TS-E 24 is my favorite lens in my entire bag, though, in full disclosure!
I've not had much luck with the f/1.4 24mm II. I struggle to see the worth of F/1.4 at this focal length, where you're typically going for maximum depth of field, and neither of the two copies I've tried were that sharp around the edges. Night-time photography, maybe. It's a lot of extra money for a few extra stops of light gathering capability, and you do after all have a 5D3 where you can bang up the ISO if you need to.
To echo Arkarch, steer clear of the 16-35 or 17-40 offerings. They're just not sharp enough for landscape use, and the latter is due an imminent update.
For two lenses, the Zeiss 21 and 24-70 II is a superb combination for landscape and general use. They're amongst the very best lenses you can buy for a Canon. You should be budgeting for a decent lightweight tripod, a ball head (I use Gitzo and RRS for this), and a selection of filters. You will need a circular polarizer (B+W XS-Pro in 82mm for both lenses is good and low profile) and a selection of graduates from Lee or Cokin. Decent tripods and filters cost more than you might think, so consider it when you're budgeting. Hope this helps.
