Folks are drawing much clearer lines as to what is a "superhero" than the history of the genre allows for.
Superman, the original Sandman, and The Spectre were all superheroes.
I think you're profoundly missing the point.
The history of the genre is irrelevant. Totally irrelevant. You want more precise definitions from people (not just me, I see), but that's not how it works. This is about people's opinion re: the best superhero movie. They don't need to consider the "history of the genre". They just need to consider what makes a "superhero movie" (and note movie specifically - not TV show, not comic book, etc.) great to them.
And to me, if we're asking this from a genre perspective, imho, it's very important that movie really
embraces that
genre (superhero movies in this case), rather than trying to subvert it, or rather than just making a movie that's basically another genre but which has superheroes in it. Like if it deconstructs it, it'd better also reconstruct it (I think you can argue Spider-Man: No Way Home does both - not in depth, but to some degree - for example - in fact you could probably also argue that about Into the Spider-Verse).
As I said, I think the question of:
A) What's the best movie which [technically] is about a superhero?
and
B) What's the best "superhero movie"?
Are completely different questions with different answers. They don't have to be for everyone, but I think it's entirely valid to consider that. And AFAIK there's never been an OG Sandman movie, or Spectre movie, so they can get stuffed, frankly. And for my money, The Dark Knight, whilst a really cool movie, is basically a crime movie with some slightly goofy elements. And that's no accident! It was much intentional and conscious, they actively eschewed superhero/supervillain stuff. No Joker gas, no Smilex, for example, just bombs and guns.
(Also I have an alternate theory of superhero movies which is that they can be identified as movies that make my wife go "Ugh I'm not watching that" - note she's fine with the Nolan and 2022 Batmans, Deadpool 1 (but not 3), GotG, and a few others, so these must not be "superhero movies"!)