Foundation Series
It is absolutely without compare. But it's a quadrilogy. I don't know if I've told this story here or not but after the trilogy, I was upset because we were left hanging so I wrote Dr. Asimov a letter asking him to please write a 4th book tying up all the loose ends. I warned him that if he didn't and I died first, I'd come back and haunt him. So I get this card from Dr. Asimov asking me not to die and not to come back and haunt him and informing me that he had written a 4th Foundation book, gave me the word count and told me he expected me to buy it in hardback. I did.