I don't often consider that 2 of the biggest gaming trends, zombie/survival and battle royale, came out of Arma mods. Arma3 is probably my "desert island" game, it's flexible enough to be a FPS, a simulator, a RTS, and much more.
There was an Operation Flashpoint mission where I was supposed to be waiting for an officer to arrive in a Jeep. I waited like 10 minutes and he never came. I walked down the road and he had crashed the Jeep and died. That to me was an important moment in gaming, it was a scripted event but in the game world, and the AI had some randomness to it. That is the sort of gameplay experiences I enjoy, when a mission isn't the same every time and the open world truly feels like anything can happen.