This is Nick's final statement to Gatsby before Gatsby is killed, and Nick says he's proud of himself for saying it because at other, public times Nick does not defend Gatsby. This quote from The Great Gatsby, along with the one in chapter 1 in which Nick says "Gatsby turned out all right in the end" show Nick's alliance with Gatsby and his dreams. The irony, of course, is that Nick only privately condones Gatsby, never publicly--a kind of cowardly admission.
The "rotten crowd" to whom Nick is referring is, in general, the bourgeoisie, or established rich like the Buchanans, Tom and Daisy, who run away from their problems and hide behind their money.
Nick points out that even a liar and criminal like Gatsby is better than the other liars and criminals because of his boyish romantic dreams, because his desires are focused on ideals (like love) instead of materialism, and because he did the honorable thing in waiting and sacrificing himself for Daisy, his holy grail.
In Chapter Eight of F. Scott Fitzgerald’s novel The Great Gatsby, Gatsby reveals to Nick the full extent of his history with respect to Daisy Buchanan, his one true love – or obsession. She was, he tells Nick, “the first ‘nice’ girl he had ever known.” Gatsby’s problem, of course, was that he came from a very modest background and could not hope to compete for Daisy’s hand in her world of affluence and conspicuous consumption. She was aristocracy; he was proletariat. As he describes the context under which he finally consummated his love for Daisy, he reveals more about his background:
“However glorious might be his future as Jay Gatsby, he was at present a penniless young man without a past, and at any moment the invisible cloak of his uniform might slip from his shoulders. So he made the most of his time. He took what he could get, ravenously and unscrupulously—eventually he took Daisy one still October night, took her because he had no real right to touch her hand.”
Gatsby, unscrupulous though he be, knew he wouldn’t measure up socially and financially to Daisy’s world. That, however, didn’t stop him from committing his life to the accumulation of wealth so that he could at least position himself geographically and emotionally closer to the object of his obsession. He knows, however, that the gulf between them is insurmountable. It is Nick, however, shorn of his idealism and naivete, who assures his neighbor and “friend” that wealth and class are two very different things. Talking with Gatsby late in this chapter, the two are preparing to go their separate ways, Nick preparing to catch a train to the city. Assuring Gatsby that he’ll “call you about noon,” Gatsby suggests that “‘I suppose Daisy’ll call too,’ to which Nick responds, “‘I suppose so.’” Nick then describes the scene as follows:
“We shook hands and I started away. Just before I reached the hedge I remembered something and turned around. ‘They’re a rotten crowd,’ I shouted across the lawn. ‘You’re worth the whole damn bunch put together.’
“I’ve always been glad I said that. It was the only compliment I ever gave him, because I disapproved of him from beginning to end.”
Nick is pointing out what the reader has been able to see throughout The Great Gatsby: The wealth and status enjoyed by the Buchanans and those with whom they associated was a thin veneer beneath which was a foundation of moral decay. Character is more important than money, and Gatsby, if nothing else, had character.