In addition to the wonderful answer from the editor above, Nick's best quality is his language (style and point of view).
Let's face it, The Great Gatsby is one of the great American novels because of Nick's authorial voice, which is both speaking for Fitzgerald and to spite him. Fitzgerald wisely splits himself, giving Nick his words, and Gatsby much of his action, and he uses Nick's eloquence to construct both himself and Gatsby. I don't know of any other novel in which a narrator so cleverly and beautifully filters another, much more memorable character entirely in flashback.
So says Enotes:
Through this first-person (“I”) narrative technique, we also gain insight into the author's perspective. Nick is voicing much of Fitzgerald's own sentiments about life. One is quite simply that “you can never judge a book by its cover” and often times a person's worth is difficult to find at first. Out of the various impressions we have of these characters, we can agree with Nick's final estimation that Gatsby is worth the whole “rotten bunch of them put together.”
Chapter one sets the tone for style. The novel is perfectly structured: chapter one mirrors chapter one, as both mention fathers and fatherland. Not only that, but Nick frames the novel so wonderfully in context of time and place, archetypal imagery, and socio-economic barriers. In short, Nick's voice allows us to read the novel from multiple perspectives.
Nick sets himself up to be the alazon narrator (self-righteous hypocrite), as the previous editor so eloquently stated. In this way, the reader trusts Nick and, like him, we equate ourselves with Gatsby. Not to mention, Nick's language is so full of imagery and wonder (in bold):
...fifty feet away a figure had emerged from the shadow of my neighbor’s mansion and was standing with his hands in his pockets regarding the silver pepper of the stars. Something in his leisurely movements and the secure position of his feet upon the lawn suggested that it was Mr. Gatsby himself, come out to determine what share was his of our local heavens.
I decided to call to him. Miss Baker had mentioned him at dinner, and that would do for an introduction. But I didn’t call to him, for he gave a sudden intimation that he was content to be alone—he stretched out his arms toward the dark water in a curious way, and, far as I was from him, I could have sworn he was trembling. Involuntarily I glanced seaward—and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock. When I looked once more for Gatsby he had vanished, and I was alone again in the unquiet darkness.