Silent Hill 2 was the first time where I properly realized just what the artistic potential of the medium is. I think what it made me realize how writing and art design can elevate a game. It had this really emotionally interesting concept, about a man coming to terms with the guilt of killing a terminally ill family member. And, like, you could easily see a book or a film working with that premise. You could even see the type of more narrative driven style of game, say a visual novel or a walking simulator using it. But to explore a story as ambitious as that in the framework of a survival horror game with all the gameplay conventions customary to it genuinely made me realize that you can fit narrative and emotion into a perfectly conventional action-adventure gameplay model.
Deus Ex on the other hand changed the way I look at gameplay. The way level design, character customization, equipment loadout and scripting all work together to create a game that gives you a large number of options for each objective and is able to anticipate and react to most of them while maintaining the integrity of its plot progression and structure. Which made me realize that games don't have to sacrifice freedom for structure or vice versa, but that there is sort of a golden middle that offers just the right amount of both.
The most recent one was Death Stranding for me. It made me think about the way games treat action an combat. DS has all the mechanics of an open world, third person action game, but it never tries to play like one. It has combat and action setpieces, but they punctuate the gameplay, rather than being the foundation of its gameplay. And it made me think about the sort of abstract nature of action in games. Take a game like Red Dead Redemption 2. Very somber tone and cinematic presentation. But in almost every movie, even the most wild and over the top of action movies, the number and scope of gunfights in RDR2, much less their bodycount, would seem downright farcical. No movie would ever even try to maintain the tone of RDR2, while cramming so much action into it. Meanwhile a playthrough of DS will contain about half a dozen climactic bossfights, a handful of combat encounters with bandits you come across along the way and that's pretty much it. You are very likely to finish the game without having killed a single human enemy. Considering Hideo Kojima has a claim (albeit an extremely shaky one) of having invented stealth games he's obviously always at least questioned the role of violence as the core gameplay principle of most games, but DS takes it even farther by taking all the mechanics of an open world action game and completely recontextualizing them in a way that deemphasizes violent conflict.