I think System Shock 2 is this for me. That said I only played enough of it to get frustrated and quit, which was barely the first level, and I knew “the twist” going into the game so that probably lessened the game’s impact. This was early enough in the Immersive Sim genre that a lot of it’s conventions and quality of life improvements hadn’t been implemented. The level design of the early areas didn’t have a lot of logic to them and so it just annoyed me and caused me to disengage with the game. I would have to go back and play it again to maybe give a more articulated response, since it was so influential on the rest of gaming.
Also Destiny 2. This game has a lot going on, an incredibly deep well of content and an incredibly complex gun and abilities system that, 10 hours into the game, I realized that I would have to sink much more time into if I wanted to get anything out of. The entire time I basically knew that the game’s gear system was way too complex for me to grok unless I could spent 100 hours on the game. Destiny’s big attraction, the raids, would take me much more time to do and require me to get in with a group of people that would take on a noob so they could onboard with the game. It just feels so inaccessible unless I dedicate so much time to it. I think the world building, level and world design, and characters are all really well done and amazing. You can, i think, pretty much engage with this game on a lore level and never have to actually play any of it.