Definitely the best of the series -- honestly a pretty damn wonderful co-op puzzling experience. It's different in tone and execution to, say, the co-op half of Portal 2, and very nearly lost me in the final 15 minutes or so, but has scratched that itch better than any other game I can immediately think of. Very pretty too, especially for a game of this kind, with a much more distinct aesthetic than the second without falling into the hyperstylized goofy trappings of the fourth.