I think the only way to answer this is to look at some stories that have held my attention. I’m going to look at each in 4 particulars Plot, Characters, and Science, why it holds my attention. These are of course my own (potentially assinine opinions) and I apologize in advance if you find your ox has been gored
I Robot by Isaac Asimov
Plot: This doesn’t really have an overall plot per se, but one in each story. This is unsurprising as it a bunch of shorts tied together. Each story is itself is essentially a mystery and I find their resolutions in general quite Satisfactory (but the final story that I have never enjoyed)
Character: Honestly the human characters are cardboard. Susan Calvin feels like a classic scientist character in drag. Donovan and Powell who show up in several are more foils for the story line. The real characters here are the robots like Robbie and figuring them out is the whole point of each story.
Science: The Robot positronic brains are sheer handwavium. Some of the resolutions do involve some actual science. That said the positronic brains are VERY self consistent and that adds to a feeling of reality.
Why I Like It: the pleasure of it really comes from how the puzzles are solved. These are almost more Mystery than SciFi at some level
A Canticle for Liebowitz by Walter L Miller
Plot: There are essentially 3 sections spread out over a period of several thousand years. Each section has an internal plot. All three sections ultimately lead to the final resolution of the plot
Character: Each section has one main character. These are well rounded (though ALL male as they are Monks or abbots of the Monastery for St. Liebowitz). Given their time scale they exist ONLY in their section, although their actions (and to some degree personalities) affect the later sections of the story
Science: This is a post nuclear war book. The effects of the radiation are wrong, though they would not necessarily have been thought unreasonable when these stories were written in the 1950s. Other science (as society recovers) makes sense though is not directly influencing the plot
Why I Like It: The story arcs are fascinating with the overall arch being rather disturbing. Mr. Millers prose is downright luminous in places and his plotting and bits of humor (winking at everyday things we understand but the characters don’t) helps offset the sombre tone of the overall arch of the story. Even though this is three separate stories the work to tie them together is elegant. This well deserved the 1961 Hugo when the Hugo meant something
To Say Nothing of the Dog by Connie Willis
Plot: This is a time travel story where an apparent time paradox must be resolved or history as we know it may just disappear. It is also a closed room Mystery AND a bit of a romance all at once with most of the action occurring in the late Victorian Age. And it successfully wraps up ALL these pieces and (mostly) sets the time stream back to its proper state. This is a tour de force of plotting
Character: Our two main characters (Ned Henry and Verity Kindle) are very well fleshed out. The secondary players also have decent motivations though are though sometimes intentional stereotypes (e.g. a totally distractible Oxford Don)
Science: It has time travel, which in all honesty, appears to be more in the realm of fantasy than SciFI (even though it has been a staple since H.G. Wells The Time Machine). That said the rest of its science mostly makes sense, and other than one critical biology issuethe science is not critical to the plot. The time travel is self consistent (although we are in the process of learning about it in detail) but we don’t get David Weber like info dumps but learn the about the Time travel as we go along.
Why I Like It: This is just an absolute romp. You are swept along by the mystery and by trying to resolve the change in the time stream that is introduced. As I said the plotting is genius level, the mystery is very like a good Poirot story. There are little details you may miss in the first reading that on second reading you wonder how you missed it. Again a worthy Hugo award winner
That’s probably more than anyone wants to read. Looking at it (and other stories I return to time and again) I am mostly attracted by good plotting. I admire additional character work and language. I am willing to suspend disbelief if the “science” is self consistent. I will tolerate massive violations of reality such as time travel, matter transports (looking at YOU Star Trek) and other massive violations of basic science. I’d love to see other people look at their favorites in a similar light