First: I totally agree with @c-m-woodworks.91222 regarding being more loyal to creators that companies.
Second: In my opinion Marvel changed the superhero comics world by adding more realistic characterization that is the basis for today’s comics. It is hard to pinpoint specific “groundbreaking events” like Watchmen or All-Star Superman as the “groundbreaking events” took place in “normal continuity” like Batman: Year One in Batman 404-407. Also, “groundbreaking” can be interpreted in different ways.
Black Panther
First Black (African not American) Superhero Fantastic Four 52 1966
First Black member of a major superhero team, The Avengers Avengers 52 1968
Spider-Man
First Anti-drug Story (printed without the Comics Code Authority Seal) Amazing Spider-Man 96-98 1970 predating GL/GA 85-86 1971
From the article above:
DC Comics was actually more pissed than anyone else, as Carmine Infantino (DC’s Executive Editor) was waiting for the new Code to go into effect so that DC could do their OWN anti-drugs story, and they were irked that Marvel just went out and did theirs without Code approval. DC’s came out later in 1971, with Green Arrow dealing with his sidekick, Speedy, becoming addicted to heroin…
Iron Man
Demon in a Bottle Saga a story of Alcoholism Iron Man 120-128 1979
Everything Else - Sturgeon’s Law
”90% of Everything is Crap” - Theodore Sturgeon
What are the great stories/runs that are the 10% that you love? Keeping with Marvel by creators:
Fantastic Four - There are four great runs over their 60 year history and even those have clunker issues/arcs along the way. Early FF is truly a slog to read through but gets great about 30 issues in.
Stan Lee & Jack Kirby
John Byrne
Mark Waid & Mike Wieringo
Johnathon Hickman & Company
Was all of Batman’s 82 year history truly great? Not really. Were there great creators like Finger, O’Neil, Adams, Aparo, Miller, and others who contributed great ideas and stories? For sure! Are there modern masters that people love or despise? Sure! Scott Snyder or Tom King? I think they both did good and not so good work with the Dark Knight and I’m sure there are fans who love them and hate them.
Creators Forever!
Companies, not so much.