What do you consider the best Superman stories? That's a loaded question for me, I have a lot that I consider bests, each for different reasons, showing different aspects of Superman. One of my favorites is "Superman Joins the Circus" from Action Comics #7 (1938) by Jerry Siegel and Joe Shuster from the Golden Age. A rather timeless tale. Superman saves a man's circus. Clark interviewed a man losing his circus, "Poor brave old man! Faced with bitter disappointment and certain defeat, he yet has the courage to keep an optimistic front! A guy like that deserves a break... and that's just what I'm going to give him." Superman saw how badly the business owner needed his help, and stepped up to help. Superman performed in his circus, attracting new paying customers. Even in the early days Superman was actually working within the system to help and promote change.
Another favorite is the heart-warming "The Girl Who Didn't Believe In Superman" from Superman #96 (1955) by Bill Finger and Wayne Boring from the Silver Age. Another rather timeless tale. Superman restores the sight of a blind girl and reunites her parents. This was a adaption of the Adventures of Superman TV episode "Around The World With Superman" written by Jackson Gillis.
Another favorite is the very different action-packed "The Four-Armed Terror" and "The Big Boom" two-parter from Superman's Pal, Jimmy Olsen #137 and 138 (1971) by Jack Kirby from the Bronze Age. Jack Kirby's entire run in the Bronze Age is amazing. Mokkari and Simyan from Apokolips, sent to Earth by Darkseid to replace the success of The Project (DNA samples from the government facility) with the Evil Factory from Apokolips. The Four-Armed Terrors (literally hatched from eggs) battles Superman and the Guardian, and tries to destroy The Project building and Metropolis. Morgan Edge, being told to flee the city by his Intergang contacts. Superman saves millions of lives.
Another favorite is the "Super Menace from Metropolis" by John Byrne of the Modern Age. That is a very George Reeves style episode title. That title in fact is from the Silver Age. Lex Luthor manipulates the solar energy of Superman scientifically. This is another timeless tale. A recurring theme. Again, Luthor thinks he's more intelligent than Superman and Luthor's undoing is his condescending assumption that Superman thinks with his fits. Superman figured it out and saved Metropolis from being destroyed, he's not an idiot, he's in fact very smart to have been able to figure this out, but Luthor still got away with it legally. And I also love how Maggie Sawyer, head of the Special Crimes Unit of Metropolis, got in Luthor's face. A real solid issue. It even had a little Lois Lane, it had everything in it that made Byrne's run work. You had Clark, you had Superman, you had a fight with a villain, you had it coming back to Lex Luthor, You had Luthor slipping thought a legal loophole at the end. You had some development with the supporting cast, some subplots in there. Really and truly, one of the strongest issues of Superman. It's a compelling story. It ties into recent events, it sets up new events, but it can be read completely on it's own and readers can get straight into it. I love that opening page with Clark Kent facing three skeletons, Lois, Jimmy and Perry, in an elevator as his x-ray vision goes haywire.
And the cover is a modern age version of an over-the-top Golden Age/Silver Age cover. That cover draws you in. Cat Grant walks up to Clark to introduce her son and Clark is shocked to see her and the whole Daily Planet staff suddenly nude as his x-ray vision goes haywire again.
Then Clark's glasses start to melt by his heat vision going haywire. His super-hearing goes haywire and he hears everyone on Earth at once, including Swamp Thing, etc.
My favorite Superman team up is Superman and Green Lantern in "Last Stand" from Superman #14 (1988) by Byrne.
My favorite "elseworlds" type of Superman comic is The Incredible Hulk vs. Superman (1999) company crossover by Roger Stern and Steve Rude.