Of coarse there is no simple answer. The answer depends on your budget and what kind of DSLR body you have. I've built a list of DSLRs and a list of wide-angle lenses. In keeping with my theory that your wide-angle glass is the most important piece of equipment a real estate photographer owns I've started to build a wide-angle lens table first.
Each lens has a link to check the price and I'd like to give the lens a rating link in the center column but I have to find way to give all the lenses in the list a rating. This may be a mixture of lens rating sites... haven't decided yet. My goal is to bring together all the information one needs to know to make an intelligent decision on which wide-angle lens to buy.
The poll that I have on the lens page indicates that the most popular lens with PFRE blog readers is the Canon 10-22mm.