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.
Fuck Halo 1. It was the harbinger of things to come.
The beginning of the normalization (and broification) of
culture, which merged most of the worst elements of jock "normal" with nerd "normal."
That and em dollar sign sinking its tentacle into gaming because if it wasn't for Halo 1, the Xbox would've been a complete dud. We live in the timeline where the Dreamcast floundered while the Xbox survived.
It would have probably been for the best if the Xbox was a dud and all that energy and development effort went into other consoles/companies.