+1 on the TS-E 17. It was made for interior shots. The 16-35 f/4 IS may have more uses, but nothing does interior shots better than the TSEs.
That all depends on whether planning to use a tripod whenever light levels are low. IS would be a winner if needing to take shots hand held where a tripod would be a problem, or program does not allow.
I'm happy with my 24/2.8 IS for interiors, and like the focal length for it's fairly realistic view of the space, whilst still including a good part of the interior. Where a wider field of view is more important, then I have been looking at the Canon 20/2.8, whose curvature of field may help for interior shots, and the Zeiss 18/3.5 as mentioned by others. Both of these have a depth of field scale, ideal for capturing interiors.
Alternatively, the Sigma 12-24 has a depth of field scale, but lacks IS. If relying on autofocus, or making your own judgements then I concur with those who have suggested the 16-35 IS. The slow shutter speeds possible with IS will allow stopping down to keep the whole room in focus, whist keeping ISO down to a reasonable level.
Shutter speeds of half to one second can sometimes achieve the blur of people or other moving objects sometimes seen in architectural shots. However, where there is opportunity to use a tripod, longer exposures can do this more reliably.
Let me know how you get on, I'm looking at these options for myself. I'm hoping that Canon continue their roll out their IS primes at Photokina in September. Meanwhile the new 16-35 IS seems the obvious answer for full frame, assuming that a tripod cannot always be used.