@BrandonInOhio
Curious to see what the response will be to being forced to choose just one story.
Sorry, I refuse to choose just one story or series or universe.
I'm 78 now and was – still am – an avid reader since my early teens. While I haven't saved all pulp magazines and paperbacks I've bought over more than six decades (I got rid of thousands just to get space for more), I still have more than 8000 printed books, over 5000 eBooks and over 14000 story chapters in HTML, downloaded from here and other sites.
As genres I prefer SF&F, Western, Crime, General Adventure, some Historical Fiction and even Romance, but no Horror.
Many of those thousands of books I've read twice or even more times.
While nearly all older books I own are translations to German or originally written in German, I switched to the English originals in the early eighties. Since about 2000 I tried to get the original English versions of my most cherished books.
With this as background information, my taste in stories changed over the years, I found books I loved reading back then now rereading them not so compelling.
Given the vast amount of stories I've read I can't select just one story or series or universe.
If someone insists and tries to force me to select only one, I would need to look into all possible candidates and thereafter maybe I'm able to select one. Even if I would do this full-time, ask me again in November to get your answer.
HM.