It's hard to beat the Canon EF-S 10-22mm USM... one of the best in the ultrawide class. I use one.
I also have a Tokina 12-24/4... if shooting with manual flash this or one of the other Tokina with a non-variable aperture is a lot easier than the zooms with variable apertures, which is the case with most in the ultrawide class.
Canon also is now offering EF-S 10-18mm STM IS, a real bargain at $300 list price. Seems nearly as good as the older 10-22mm. Note that the cheaper lens has IS. It's STM AF is a slight downgrade from USM for still photography, but STM can be better for video... OTOH, the 10-22mm is a bit better built (a little less plasticky).
You'll have to be careful with any of these lenses. They tend to have pretty strong distortion effects at the widest settings. The wider the focal length, the more perspective and anamorphic distortions you'll see. That's just the nature of very wide angle optics. The Canon 10-22mm is about as well corrected as possible. Same with the Tokina 12-24mm. Any lens you are considering you should look up test shots done with it (
http://www.the-digital-picture.com/ has tested many) and check for barrel/pincushion... Or, even worse, look for "mustache" distortion, which is harder to correct in post-processing (for example, the Samyang/Rokinon 14/2.8 has a lot of this type of distortion). Also watch out for edge/corner softness. It can be a problem with ultrawides, particularly when used "wide open".
One solution to minimize distortions is to take multiple shots using a less extreme focal length and then combine the images using a panoramic software.
The only f2.8 lens in the ultrawide category is from Tokina... Their 11-16mm has been around for a while now (two versions, but little difference between them in Canon mount)... it's quite sharp, but also prone to flare. And to get f2.8 you give up much range of focal lengths. Tokina has a new 11-20/2.8 coming out, though, to supersede the 11-16mm.
Sigma offers an f3.5 (non-variable) 10-20mm, but it's rather expensive and one of the largest/heaviest in the ultrawide category. Not to be confused with their cheaper 10-20mm with variable aperture.
The things is, you hardly need f2.8 or even f3.5 for this type of photography. You'll almost certainly be stopping down for best sharpness and adequate depth of field anyway. A big aperture would be pretty low on my feature wish list, for this type of lens. Use a tripod and smaller apertures/slower shutter speeds will not be a problem.
Use a flash. Actually for interiors multiple flashes would typically be better. A friend of mine is one of the top architectural photographers... He arrives with a van full of lighting gear and sometimes spends 8 or 10 hours setting up for a few shots. (
http://luhnphoto.com/architecture/). Jeff loves digital since, for example, he no longer needs to balance interior and exterior light. Now he'll just take two shots... One exposed for the interior, the other for the exterior... And then combine them in Photoshop. Much, much easier than the days of film when you had to greatly boost interior lighting, as well as change out all the tungsten interior bulbs with special daylight bulbs, to get the inside to balance with the scene out the windows.
One of the shots on his website (room with piano), even without trying to balance exterior, required something like two dozen carefully balanced light sources to make for a very "natural" but well-lit interior Looks simple, but sure isn't!