If I go with unique experiences as a criteria just to mix it up a bit I have a few

Playing xwing with a flight stick is really fun and one of my first gaming experiences outside of nes and arcades. Just really fun fast paced fighter combat. A close second would be everspace or Chorus on a controller.

Playing one of the big multiplayer arcade cabinets like Simpsons or X-Men was the original couch co-op with whoever was at the arcade not playing pinball or pool.

Beating your first boss in a dark souls or elden ring game feels like a big accomplishment, sometimes even more than the later ones when you’re character gets more powerful.

Going from small groups with friends to raids in an MMORPG feels like starting a whole new game, EverQuest and wow were big ones for me.