There are so many... I'll just go with a couple
Fallout 3 Wanderers Edition always comes to mind for making that game so much better to play and more stable than the base game. I wouldn't replay FO3 any other way
Elden Ring Seamless Co-op is what ER co-op should have been to begin with. God it's so good I put another couple hundred hours in to replay every inch of ER+DLC with a friend.