As was mentioned before, I'd stick with the middle of their catalog if you want to hear what made the band famous, with those albums being Dark Side of the Moon, Wish You Were Here, Animals, and The Wall. I'd then check out Meddle, as I find this to be the best of their pre Dark Side material. You can listen to anything earlier in any order, though you'll probably find The Piper at the Gates of Dawn to be a bit weird (same with Ummagumma). Finally, check out the post The Wall material. Pretty much all of their stuff is good, but their music varies greatly from their inception to their last album.