I'll take in the
Rock category, my favorite of Pink Floyd's numerous masterpieces...
Meddle.
Jammed for time so I can't offer much of a write-up, but Meddle is... amazing. To me it represents the Floyd's perfect marriage of their prog and more mainstream elements. Set the table for them to take over the world with Dark Side of the Moon, and like has been said before, as a complete listen, will turn any casual fan into a disciple.