If I had to settle for one film, then it would likely be Tri-X or HP-5. As it is, I, like David, have a few that I'm used to: FP-4, APX-100, APX-25 (only 6 rolls left!).
If one is shooting professionally, then settling down to one or two films is maybe a good thing. I personally shoot for fun (and the odd print sale or commission). Trying different films is fun, so I indulge myself.