Gardner Dozois stands in 1991, surveys the previous thirty years of science fiction stories, and chooses these twenty-six. He makes it clear in the introduction that he isn't trying to trace the genre's history, isn't choosing stories to represent different subgenres, and isn't trying to showcase any of his favorite authors. He picked stories he enjoyed reading. It's a good thing to be an editor.
Here are my favorite four of the editor's favorite twenty-six.
Damon Knight's "The Country of the Kind" examines the life of a lonely man who keeps reaching out for others. Something always gets in the way. This story may have influenced Anthony Burgess' A Clockwork Orange
Gene Wolf's "The Fifth Head of Cerberus" "echoes of Proust and Dickens and Kipling and Chesterton." It's a family history of a closer-than-usual family in business together.
Howard Waldrup's "The Ugly Chickens" follows an investigator trying to prove that an extinct bird may still be alive. What he finds makes him truly thankful.
Lucius Shepard's "Salvador" puts us inside the head of a soldier fighting the enemy in a South American jungle--with a little help from his little friends.
Gardner Dozois' taste in science fiction works for me. These stories are all interesting and most are fun to read. A couple of them take you to a dark place. So watch out for that.