Can you identify a quote from "Winter Dreams" that describes the plot?

Interestingly enough, I think that the last line of the story best describes the plot of the story. Dexter is sitting alone in his New York office, looking at the skyline.  He is dejected and broken after hearing about Judy Jones, who has now become Judy Simms.  He realizes the truth that lies at his being with the closing quote of the short story:

Long ago, there was something in me, but now that thing is gone. Now that thing is gone, that thing is gone. I cannot cry. I cannot care. That thing will come back no more.

This quote describes the plot of the story.  It is a statement about how all pursuits that are rooted in the world of the shallow are bound to be fruitless.  It is a statement that describes the plot of individuals who engage so much effort and endeavor in the world of "the now" and refuse to see anything larger or beyond it.  This becomes a descriptor of the plot because such a statement summarizes both the characterizations and the sequence of actions in both Judy and Dexter.  The final words of the story are a description of the plot because they speak to how individuals are bound to feel hollowness in that which is rootless.  This becomes the plotline of the story and the backdrop of the 1920s.  For characters who do not speak much larger outside of themselves, Dexter's words at the end of the story are some of the few lines of reflection.  They speak of a reflective quality that is not evident in much of the story.  It is for this reason that the words are significant at the end of the story and are able to summarize the plot quite well.

What quote best represents the story "Winter Dreams"?

I assume you mean a quote from Fitzgerald's story that encapsulates the theme. If so, I would choose this one: "Dexter was unconsciously dictated to by his winter dreams."

The quote reflects Dexter's lifelong ambition to rise beyond his lot in life of a lower-class man who lusts for the "good things": money, beautiful women, and respect. As a caddy for the rich, Dexter sees what other people have and wants those things too.

On one level, Dexter gets those things. He succeeds in business and briefly acquires the fey Judy Jones. However, the realization of his dreams proves hollow. Money doesn't comfort him, Judy leaves him (though he never really had her), and his business life is dull at best. He longs to return to the promise of his "winter dreams" rather than the reality with which he now lives.