This is a good question. Let me answer in three different ways.
First, on the most superficial level, Nick thought that Gatsby was great on account of his showmanship and wealth. Not only Nick, but also all were quite taken in by Gatsby.
Second, on a deeper level, Nick saw behind the exterior and he saw a glimpse of Gatsby's heart. In other words, Nick knew why Gatsby did what he did. All of his achievements and external pomp was for the sake of love. He wanted to impress and gain the love of Daisy. In this sense, he was singular. In chapter six, we get this glimpse:
The truth was that Jay Gatsby, of West Egg, Long Island, sprang from his Platonic conception of himself. He was a son of God—a phrase which, if it means anything, means just that—and he must be about His Father’s business, the service of a vast, vulgar, and meretricious beauty. So he invented just the sort of Jay Gatsby that a seventeen year old boy would be likely to invent, and to this conception he was faithful to the end.
Finally, Nick realized that the others were far worse than Gatsby. They might seem better externally, but they were deeply superficial and arrogant. In this sense, Jay Gatsby was far better.