My own personal favorite Tiptree story is "The Screwfly Solution." In this story a sort of psychological plague has broken out in various parts of the world where men are murdering women wholesale. Tiptree introduces us to (and makes us care about) one particular family. In 21 pulse-pounding pages Tiptree gives us the stunning macro-story of the fate of humanity in the face of this terrifying "plague," along with the heart-wrenching micro-story of its effect on one family. It is a masterpiece of economical storytelling, and no SF story has an ending which packs a bigger wallop.
My (close) second favorite story in the book is "A Momentary Taste of Being." In his introduction to the book, John Clute writes of this story: "...word-perfect over its great length, and almost unbearably dark in the detail and momentum of the revelation of its premise...[it] may be the finest densest most driven novella yet published in the [science fiction] field." I can tell you it is my all-time favorite novella. The story concerns a space mission, a desperate attempt by humanity to find a habitable planet (for colonization) to relieve some pressure from a horrendously overpopulated and polluted Earth. The pressure in the story just builds and builds to a climax as intense as any you are likely to experience in fiction.
I think "Love is the Plan the Plan is Death," a story of alien love, is a one-of-a-kind masterpiece of style. Not everybody agrees. Gardner Dozois in his excellent and mostly laudatory essay, "The Fiction of James Tiptree, Jr.," writes of this story: "I can never read [its] galumphing, ungrammatical, childishly-rapturous narration without hearing it in the accents of the Cookie Monster...." Tiptree herself, in typical self-depreciating fashion, described it as being written in "the style of 1920 porno." I think the highly unusual style helps us understand and feel the true alien-ness of the viewpoint character, and I believed totally while I was reading. As John Clute writes, "...[it] has a juggernaut drive, a consuming melancholy of iron, a premise the author never backed away from...."
In "Houston, Houston, Do You Read?" three astronauts return from a trip around the sun only to find they have somehow been transported hundreds of years into the future. What they find in the future, and more important, how they react to what they find there, constitutes the most powerful story I've ever read dealing with the gulf between the sexes.
In "The Girl Who Was Plugged In" a horribly-deformed young woman gets a chance at a happy life. This is another story with an unusual narrative style, and frankly, when I first read this story over two decades ago, I found it a bit disconcerting. It works for me now, though. This is a heartbreaking story, fiercely told.
One caution is that I would encourage you to read the stories in the book before reading John Clute's introduction, as Clute gives away some of the story endings in his introduction. And surprise endings are not uncommon in Tiptree stories. I am not talking about gimmicky, meaningless surprises, there for the sake of having a surprise. Tiptree's surprises often ENLARGE her stories, altering the meaning of what has gone before, increasing their power to move us. The book gets my most passionate recommendation.
She is just unbelievable, and that's all there is to it.