The Sigma 10-20 won't work on full frame (might be okay at the 20mm end only).
I also don't think you need to limit yourself to just L glass... There simply are a lot of great lenses that don't happen to have a red stripe painited on them.
For versatility, a zoom would be the way to go. The 17-40 is the cheaper choice and quite popular.
The new Tokina 16-28/2.8 is a possibility, however I haven't used it and can't really say much about it. Tokina AT-X lenses are generally very high quality, though.
The Canon EF 16-35/2.8 II is the premium choice and priced accordingly.
The Sigma 12-24/4 is the widest lens available for full frame, pays the price compromising on image qualities a bit, but goes where no other lens can.
You have to ask yourself just how wide a lens you really need. The 24-70/2.8 isn't as wide, but is top quality throughout it's range. I use one and on this "middle zoom" I want f2.8 minimum... but I don't really feel the need for f2.8 on a wider lens. Often we are stopping down wide lenses, not using them wide open. Since you have 50mm and 70-200 covered, a 17-40 might fit into your kit better.
Generally speaking, for a better corrected wide lens, you might want to consider a prime instead. I complement my 24-70 with a Canon 20/2.8 and it's very good.
Need wider? Typically, 20mm is wide enough for me. However I've considered getting a 14mm as well, for the occasional extreme wide angle shot. The Canon 14/2.8L II is super expensive. The Rokinon 14mm appears to be a bargain (also sold under the Bower and Samyang names, plus as a Vivitar 13/2.8... same lens in all cases, though). However, it's a fully manual lens... both focus and aperture. So your viewfinder will dim as you stop it down and you'll need to use "match needle metering" to set exposure on your camera. Those things might be no big deal for scenic shots, though. It also has some "moustache" distortion, which may or may not matter depending upon the subject matter... And can be largely corrected with software such as PTLens, too. Another thing, it tends to vignette fairly strongly when wide open on full frame... Some retailers actually refer to it as a crop-only lens because of that. If the vignetting is a problem, you can stop down and it will largely disappear by f5.6 - f8 (dark viewfinder, though). Manual focus is no big deal... Such a wide lens stopped down will have gobs of depth of field. Note that the Vivitar version is one of their Series 1 models, so has a red stripe and some casual observers might mistake it for an Lseries
There are an older version of the Canon 14/2.8, as well as a Sigma 14/2.8 and Tamron 14/2.8, too.... All of which might be found used for considerably less than the current Canon 14/2.8 Mk II, but they all have some degree of IQ degradation. The Canon 14/2.8 II is hands down the best image quality of any lens this wide.
The 35/1.4L is another lens on my personal "short list" since getting 5DII a couple years ago. It's not a lot wider than your 50mm, but is more to my liking than 50mm on FF. I just prefer a "slightly wide" normal lens. I've been making do with a 28/1.8 I already have (and really like on my crop cameras). But some day I will add the 35L, I'm sure. Is it wide enough for you? Again, it's not a lot wider than your 50mm (but it's a far, far better lens).
You haven't mentioned, but there are also the 24/2.8 and 24/1.4L prime lenses. The 24/2.8 is not a USM lens but is relatively affordable... the 24L is quite expensive. There's nothing in between, unfortunately. Well, maybe... Sigma made a series of f1.8 wide lenses: 20mm, 24mm and 28mm. They are not HSM, but some folks really like them. I haven't used so can't really comment.
And I mentioned the Canon 28/1.8... it's a good lens... reasonably affordable and quite compact among the Canon primes... with fast/accurate USM focus. It is great on crop, which uses the "sweet spot" center of the lens. On FF it will be a bit soft in the corners wide open, but improves if stopped down. It's also reasoably fast. To me, 28mm is sort of a "ho hum" focal length on FF... just a personal bias. I'd rather use slightly wider 24mm or less wide 35mm... but I still use the 28mm and it serves pretty well.
There are also the Tilt Shift lenses, which can be quite useful and fun for some types of scenic shots. There are two wide TS-E lenses: the 17mm and the 24mm Mark II currently. Hella expensive, though. Especially the 17mm (which I'm still drooling over). The earlier version of the 24mm, bought used, is another possibility. It works pretty well but has some chromatic aberration and doesn't enjoy the dual axis control of the newer version (that allows you to set align the tilt and shift or to set them 90 degrees out of phase with each other... the older lens has to be disassembled partially to make this change). These are manual focus lenses, but the aperture reamains fully controllable in the normal manner, so you don't have the dim viewfinder isses and hassle of stop down metering, the way you do with some fully manual lenses (see the Rokinon 14mm above).
There are also literally tons of vintage manual focus lenses out there available used at relative bargain prices, many of which are wide angles and easily adapted to use on EOS. There are some great lenses, too, though they will be fully manual focus and aperture.
There was a Tokina 17/3.5 made for a while in Canon EF mount. Earlier manual focus/manual aperture versions of this lens in other mounts that can be adapted for EOS can be found, too. There also is a Tamron 17/3.5 Adaptall2... a bit hard to find, but easily adapted interchangeable mount lens... again full manual focus and aperture.
Finally, manual focus but otherwise with full aperture control are the superb Zeiss ZE lenses for Canon. They aren't cheap, but are simply top quality, easily rivalling any L series lens IMO. Currently there are 18mm, 21mm, 28mm and 35mm wide angles offered. I think I've heard that a 25mm will be offered soon, too (it's already available in Nikon and Pentax mounts).
There are sample shots with many of these lenses in the Lens Sample Archive. Just do a search for any that interest you.
