Not sure if I could pick just one, but Meddle ranks very high on the list, along with Obscured By Clouds. Meddle is interesting because it really straddles the line between the old PF, with collections of songs, and the Roger Waters era of their "concept" albums. Of the later stuff, I like Animals and WYWH the best, although DSotM is a classic, and I've spent countless hours listening to that album. The stuff after The Wall I find less interesting, although to be fair I've not delved too deeply into that part of their catalog.