The City and the City, but it wasn't without its flaws. And I didn't even get all the way through The Kraken and Perdido Street Station - both were good, but too sprawling and unfocused for my tastes. Embassytown is the real deal.
I'm not going to give too much away about the plot, because much of the enjoyment comes from the unexpected twists and mounting drama. It's set very far in the future, and a colony of humans has been on an alien planet for generations, in a small enclave called Embassytown. The aliens are the Arekei and are treated with the utmost respect and called Hosts. They in return supply living biological technology from vehicles to homes and power stations. All very weird and interesting, but it's the communication between the species which is the real focus of the book.
The Arekei cannot lie, and this throws up some fascinating ideas. For instance, some of the humans have become living similes, so the aliens can use them as rhetorical devices. The protagonist Avice is "the girl who was hurt in the dark, and who ate what was given to her." This had to literally happen to her, so she could become a simile. It doesn't make a lot of sense to humans, but the Hosts talk about her in different ways, and this search for nuance and, ultimately, lies, becomes hugely important to everyone on the planet.
This is all a bit hard to get your head around sometimes, but I found it hugely rewarding. I saw echoes of some of the best sci-fi here: Lem and the Brothers Strugatsky in its convincing portrayal of the deeply alien; A Fire Upon the Deep and Speaker for the Dead by Orson Scott Card in the way it looks at communication between humans and the deeply alien; Dune and Asimov's Foundation series in its epic scope, its portryal of earth-shattering changes in society, and even in the rise of ambiguous prophets and gods.
Despite all this, it's not overly long and there's always a clear dramatic focus to keep you going. It's also intelligent and literary, and the writing and imagery are beautiful and affecting. This is a huge recommendation from me - the best book I've read all year.