Don't be an asshole.
I fail sometimes following that rule and I don't deny it. I struggle to become better but keep going. Of course you could extend this by "Help others, when appropriate", "Be nice to each other", "Use common sense", "Pardon others", "Don't be a d***", "Don't be a b*****" but that follows logical and would be itself a CoC. Anyway, Captain Picard said everything:
Mister Worf, villains who twirl their moustaches are easy to spot. Those who clothe themselves in good deeds are well camouflaged.