There are a lot of good candidates, but I'm going to have to go with Your Eternal Reward from Team Fortress 2. I was a Spy main, and YER just elevates all the aspects of the class I enjoyed in a way nothing else did. It let you actually be sneaky in a way normal Spy gameplay usually isn't. It's the best execution of the class fantasy in the game, and getting into the flow with YER was some of the most satisfying experiences I've had in gaming.