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.
All-in-all, We Were Here Together picks up where everyone returning to the franchise expected, and that's perfectly okay. It boasts some really clever puzzles and lays down the solid foundation on constructive teamwork. While some puzzles require a couple retries along some trial and error, and the plot feels like a misguided stepping stone to leading our two players into a derelict castle in the middle of nowhere, it's still worth hunkering down and working out your brain with a buddy for a few hours.