Okay, I"m thrilled to have finally received my 5D MK III! Coming from my 7D I have some of the longer focal lengths covered, 70-200mm f/2.8 IS II and the legendary 100-400mm L. Most of my wider lenses are EF-s Lenses like my wonderful EF-S 17-55mm f/2.8...
I need some wider glass for my 5D III any suggestions? I'm looking at 24-70 f/2.8, 24-105 f/4, or maybe primes like a 24mm or 35mm or 50mm f/1.4... any thoughts?? EF 85mm 1.8 is on my short list, but it's not really wide...
24-70 2.8 II is good and will deliver the same edge to edge sharp pics on the 5D3 as your 17-55 2.8 IS did on the 7D.
The 24-105 cost a lot less and has IS but it won't be as crisp edge to edge as the 17-55 was (or even quite as sharp anywhere).
There is the upcoming 24-70 4 IS. Perhaps it would be most similar to your 17-55 in that you get the same degree of low DOF ability and IS. Hopefully it does better than the 24-105. The MTF suggest it will be similar to the 24-70 2.8 II at the long end although not quite as good on the short end although better at either end than the 24-105. It remains to be seen.
The 24 2.8 prime is no better than the zooms.
The 24 1.4 II is very good, although if you don't need the f/1.4-2 much then the 24-70 2.8 II pretty much covers it since it's a really good zoom (the 24-105 doesn't do as well as the 24 1.4 II even at f/6.3-f/11 for landscapes).
The new 24 2.8 IS might be decent enough.
The 24 T&S II is very good and lets you adjust all sorts of things that you can't with a regular lens, it is specialized though and Samyang is coming out with one soon for a fraction of the price, maybe that one will be good (their 14mm is crazy good other than TONS of distortion).
Zeiss 21mm 2.8 is very good.
17 T&S is good if you want fancy, specialized ultra-wide work.
A 35 1.4 or 50 1.4 is nice for some low DOF work and such. I've been wary of sigma in the past but their 35 1.4 is getting amazing reviews and apparently the AF isn't bad on it. The Canon 50 1.4 has dodgy AF, an ancient and unique AF design that also is prone to breaking but it performs well optically for a standard 50mm design (an exotic-type design might do better wide open though). I'd favor a 35 1.4 over a 50 1.4 on FF, I think, but it's up to you.
50mm definitely is not wide, 35mm is only starting to get wide.