I love all their stuff, except for The Final Cut (which just makes me insanely unhappy), but my favourite three are Dark Side Of The Moon, Meddle and Obscured By Clouds.
Dark Side Of The Moon is just perfect to my mind, especially the use of slide guitar on Breathe and The Great Gig, and also I love the keyboards on it too. (Pink Floyd needed more Richard Wright on albums after it, I think).
Meddle would be worth the purchase even if it was just Echoes and One Of These Days, particularly as Echoes is my favourite song. It just has everything. I do like the rest of the album too, especially A Pillow Of Winds, but Echoes and One Of These Days are just brilliant.
Obscured By Clouds has a few moments which aren't quite as good as the rest of it, but in general I love it, especially the opening instrumentals.