Are you looking for a 'Wide Angle' lens, or just something that is wider than your 70-200mm?
Most people are recommending 'Ultra Wide Angle' lenses, but it doesn't sound like that is what you are looking for.
The EF-S 17-55mm is indeed a very good lens. Practically everyone I know, that owns one, just love it. By all accounts, the image quality is on par with L lenses. Canon seems to think so too, based on the price of it. It does have a couple issues, one being that it's prone to getting a little dust inside the front element and the IS system seems to wear out with heavy (professional) use.
The 16-35mm F2.8 L II, is also a highly regarded lens. Expensive though, and some don't like it's flare characteristics. Might still be one of the best choices.
The 17-40mm F4 L is another great lens, and at a decent price for L quality.