A 'classic' manner involves a few things.
#1: Superman is portrayed as an empathic character, who when put to choose to do the hard thing (such as not take a life) or the easy thing (take a life), he chooses the hard road. And if he makes the wrong choice, he owns up to it.
#2: The story is consistent with the theme it is presenting; be it a one-off story, a story within decades worth of continuity or its own continuity. Essentially, it needs to have verisimilitude. If the story starts off one way but then contradicts itself, it's not a classic story.
#3: The plot is imaginative, which highlights a scenario or character beat we don't usually see.
#4: Or, the plot is rather simple, but still well told with all the expectations we come to see from Superman.
The thing is, as a fan, I've seen all four elements I've listed in every age of Superman. From his Golden Age days all the way to the present. All my absolute favorite Superman stories (comics, movies, ETC) have them as well.
When all four (or the two or three that are appropriate) are in play, I'm left entertained with a smile on my face (even if the subject matter isn't always the brightest, as Alan Moore's Superman stories generally weren't).
It's why I love the titled "Superman Classic" by Rob Pratt
. It's not even a full story, just a short film, but it feels like Superman. It fulfills elements 2 and 4 for me. Is there anything more classic than Superman fighting giant robots?
By its nature, this is a very subjective topic. But those are the aspects that are always in the back of my mind as a reader, but they're also what inform me when I've written my various Superman projects... a Superman OGN, the Last Son of Krypton screenplay and a Superman pitch for the monthly gig.
But not every story from a writer is a classic. Lord knows I love Cary Bates, and he's probably my favorite long-term Superman writer, but even he had his stinkers.
I have a lot more I could discuss on this, but it's late and I want to get to bed before the sun starts coming up.
But to sum, I want a character that cares and uses his power to the best of his ability to accomplish morally hard tasks, a plot that doesn't contradict itself with unique elements, villains, supporting characters... or at the very least, tells a well-trodded plot in a fun way that leaves me smiling.
Something I'll remember years down the line for entertaining me. You do that, you have a classic.