Need Some Advise on Wide Angle Zoom for 5D Mark III

So what did you decide?

There is Canon 10-22 or Sigma 10-20 for crop bodies
There is Tokina 16-28
Tamron 15-30 is an option
They mentioned 16-35 f4L IS because it is the editors choice these days, what else you need.
If you have a big deep pocket then go with Canon 11-24 and never look back, i wish to buy this lens, but i will wait Venus 12mm f2.8 0-D.

I with the buy better glass first group. 5D3 will be around forever and a lot cheaper soon, but still, a 60D with mtiple L lenses is a better camera than a 5D3 with 1 very limiting focal length.
If you are not concerned with 2.8, get a 24-105 and a 5D3 package if you must buy a camera, but otherwise get that lense used for $500 and then get the 16-35 f4. That's a good setup.
Me personally for landscapes? I would get the 16-35 and then a 70-200 f4. I don't care about the gap as much as having reach and wide. And for now you would still have your 18-whatever that was. You would have a major boost to image quality, and THEN have a good reason to move to FF.
But even then, I would say for landscapes, if you don't already have an awesome tripod I would get that.
My 6D had issues last year before I went on a trip, I wanted a 7D2, so while it was in for repair I bought that I went on my way. I got images that I don't feel are inferior. In fact, before the 6D, I had an XSi/450D, and neeeeeded to upgrade. Then I rented a 24-70 2.8 II and realized my lenses were my "issue" (other than 400 ISO being the end of decent)
I still only have a 50L for my "gap". Doesn't bother me a bit.