This is a followup to @SorosFootSoldier@hexbear.net 's recent thread for completeness' sake.
I'll state an old classic that is seen as a genre defining game because it is: Myst. Yes, it redefined the genre... in ways I fucking hated and that the adventure game genre took decades to fully recover from. It was a pompous mess in its presentation and was the worst kind of "doing action does vague thing or nothing at all, where is your hint book" puzzle gameplay wrapped in graphical hype which ages pretty poorly as far as appeal qualities go.
So many adventure games tried to be Myst afterward that the sheer budgetary costs and redundancy of the also-rans crashed the adventure game genre for years.
I have a lot of thoughts on Doki Doki Literature Club.
The game did not invent the concept of "Story that pretends to be about cute anime girls at the start turns into horror" by any stretch, nor did it invent "meta horror" where a game purposely crashes or whatever, but what sets DDLC apart from the others, in my opinion, is its underlying sincerity.
The premise of "Anime school girls in a horror game" often results in pandering too much to sadism, the characters just become bodies to be brutalized. The narrative becomes mean-spirited, and characters are made to be purposely shallow so the audience can enjoy the violence without feeling compassion for the victims. Don't get me wrong, my monkey brain can also appreciate a slasher movie every now and then, and it's definitely more enjoyable to see a bitchy, shallow mean girl get murdered than a nice, sweet girl with complex character motivations.
Doki Doki Literature Club very much does not do that. What sets it apart is not the horror aspect, but the cute anime VN dating sim parts of it. It has real, flawed characters that could absolutely work in a "normal" VN as romantic interests, and that is what's given it its longevity. The player is made to care about the characters and the misery that befalls them is not played for spectacle, but as the more or less authentic climax of their storylines.
I could go on a lot longer but the tl;dr is that to like DDLC, you have to like anime dating sims I guess. What makes it special is not the horror, but the sincerity with which it is written despite being a horror game. Doki Doki Literature Club Plus came out on Steam a few years ago and it features a bunch of bonus stories, and none of them includes any horror, it's just a selection of cute scenes of the 4 girls interacting at school.
The song that plays during the credits really encapsulates what I mean very well. Other writers would 100% give her more "crazy stalker gf" lines up until the very end, but this song is not at all like that. It's just sincere. Monika is allowed to be a real character with real feelings and a real conclusion to her arc, and that's what makes it work and what gave it its cultural impact.