The Left Hand of Darkness. I chose The Telling, one of her more recent books, because it was a nice library copy (unlike the more dingy copy of The Dispossessed, for example), which perhaps was a mistake. Though I didn't hate The Telling, I did find it dull and lacking any innovation or interesting world building.
The Telling is the story of Sutty, an Ekumenical Observer sent to research the state of Aka. Though Sutty had studied traditional Akan language and literature prior to her trip, by the time she arrives, such practices have been replaced by an oppressive anti-religious regime bent on destroying all elements of their traditional culture. However, when Sutty is permitted to travel away from the main city, she discovers an underground world still practicing the Telling. Sutty soon becomes enraptured by their culture and practices.
The most disappointing part of The Telling is how dull the traditional religion is. It's somewhat similar to Buddhism, but mostly it includes respect, healthy living, community, etc. It's any non-religious liberal's ideal religion, and it's so obviously better than the state-controlled propaganda that there's no subtlety to be seen. Le Guin takes some long chapters in the middle of the book to describe the religion, all of which is dreadfully dull.
I often chide my students for complaining that a book is "pointless" because "nothing happens." There doesn't have to be action for a book to be great. But, in this instance, I think the lack of plot is a legitimate grievance. Sutty travels to Okzat-Ozkat; she learns about the Telling. She describes the Telling to us in detail. She travels to the special "holy" place of the Telling. That gets described too. The end. There's no climax nor any meaningful character growth. We learn early on that Sutty comes from an authoritarian religious government, but that doesn't seem to prejudice her against this religion. We also know she lost her partner, but again, it's not especially relevant.
Clearly Le Guin is a master sci-fi writer, but when I try her work again, I'll go with one of her classic pieces.