was this his best movie?

Oh, I really don't believe that's true. Ray was contracted sometime in the early '50s to write a screenplay based on a scholarly work about adult criminal psychopathology that was also entitled "Rebel Without a Cause," but the final movie had nothing to do with the book at all. The original book appears to be as close as the makers of this movie ever got to anything resembling actual research into the subject of so-called deviant behavior, however.

Ray didn't join up with his co-writers, especially the all-important Stewart Stern, until 1954. The scriptwriting team literally could not have spent "years" in doing whatever actual research they might have done, they only had a few months. From a Vanity Fair article on the making of the movie:

"'It was neither the psychopath nor the son of a poor family that I was interested in,' he [Nick Ray]said. It was only after he began developing his own story, called The Blind Run—a violent, 17-page story outline—that the idea for Rebel took shape... Ray could never have made that movie in 1954, but soon he began to work with Warner Bros. producer David Weisbart, who had been a film editor on Tennessee Williams’s A Streetcar Named Desire, directed by Kazan. At age 39, Weisbart was the youngest contract producer on the lot. Together he and Ray worked to turn The Blind Run into an acceptable story. Ray eventually came up with the solution to what he saw as the problem of the book: he made the juvenile delinquents into middle-class malcontents from 'nice' homes—they are us. Best-selling novelist Leon Uris and a former schoolteacher, Irving Shulman, wrote early versions of Rebel before Ray found his voice in Stewart Stern, a 32-year-old writer from New York, who was brought to Ray’s attention by the dynamic young film composer Leonard Rosenman."

It was basically a movie industry inside job, in other words. Never really intended to be a realistic examination of the lives of real street punks, and it shows.

(It was also a bit prophetic, I suppose, in almost inadvertently foreseeing, more than ten years before the fact, the late 1960s counterculture, which involved a whole lot of "middle-class malcontents from nice homes"-- but it took the sudden popularity of marijuana and rampant anti-Vietnam war sentiment to accomplish that feat.)

There are Hollywood legends about Nick Ray taking the young actors on field trips, of a sort, to hang out with actual JD's and try to pick up on their mannerisms, make their own contributions to the screenplay, and so on. Like almost everything surrounding James Dean, you can be sure these stories have been heavily romanticized. Considering that the film was wrapped within less than two months of production time, they can't possibly have spent more than a day or two in such dubious pursuits.

That is, "dubious" in the sense of, what could they have contributed to what we see on screen? The most realistic scenes in the whole film are when Jim Stark and the other students are cutting up, making noises while their class is on the trip to the planetarium. Not too difficult to get a bunch of young actors to play out a scene of high school students making fun of class. But in real life, the real "hoods" from their school wouldn't have been acting that way. Most of them wouldn't even have been there. If they hadn't promptly dropped out of school at age 16, they would have spent most days skipping school. Bothering to show up for a field trip to a planetarium? Not on your life.

"I don't deduce, I observe."