Let me get the criticism out of the way first. Over the years, the unending franchise of Pern books has diluted the original magic of this book. After the first two series (this first trilogy, the Masterharper trilogy) and arguably Moreta's story (worth a comfortable 4 stars), McCaffery started trying to fit Pern into a scientific jigsaw puzzle so that it would "make sense." In my view, that was a major mistake, because the glory of this first book, and what made it a true classic, is the degree to which the reader contributes to the world the author created.
Enough of that. This is the classic, and it has earned its reputation. I read this book in the late 70s. I have probably read it a dozen times since then because it is so gosh-darn easy to fall into Dragonflight... and not want to drag myself out again.
I know intellectually that Pern is a made-up universe, but emotionally it's another story. In my heart, I believe it exists. That's how absolutely "real" her world is. The background appeals to our analytical sense of "what if this happened...": forgotten colonists on a generally well-endowed planet, with this one teeny problem: a neighboring planet throws destructive spores at Pern every 200 years, and the residents create genetic telepathic "dragons" which can counter the threat. But the science is left behind, because the story starts thousands of years later, when all the backstory has turned to myth (and not well remembered myth, at that).
But lots of people can create a good world. McCaffrey created marvelous characters to fill it. Like anybody stuck in a "save the world" situation, they try to act heroic, but they fumble because they're just people.
And like the best writers, she makes them come alive with the tiny details. There's one scene, for example, that I can remember with near movie-detail imagery, even when it's been two years since I last skimmed the pages. Lessa, our heroine, spent ten years hiding out as the lowest of filthy servants. The author describes her exaltation as Lessa gets to take her first true bath in years... and how her hair refuses to lie flat, frizzing and curling while she's trying to hold a conversation. It's not an "important" scene, but it paints the background of the world in which she lives, so that we sense the way the people live... not just what they say to one another in the foreground.
I've reviewed a few hundred books on Amazon. If I were permitted only five books on the proverbial desert island, this would be among those I'd choose.