If I only want to pay for one credit score, which credit reporting company should I order it from?

I ordered my annual free credit report from all three major reporting companies (Experian, Equifax, and TransUnion), but the free reports do not include a credit score. As I understand it, the companies score your credit differently, so your scores may be slightly different from one to the next. I’d like to see my scores, but I’d rather not pay for a score from all three companies. Which one is the best one to choose?

In this case, “best” may mean that it’s the one most commonly used by mortgage companies, that it is most in agreement with the other two companies’ scores, or some other factor. (Just explain.) Factual info and personal experiences welcome!