Not sure if you've looked at Wins Probability Added (WPA), but that might be something. It takes into account the context of the situation for the player and how their contributions add or detract to the probability of the team winning that game (I think I'm explaining it right). So a grand slam by a hitter in the bottom of the ninth with two outs when the team trails by three is a huge shift in WPA, both for the hitter and the pitcher who served it up.
Here's the Fangraph's WPA leader board for relievers in 2018:
https://www.fangraphs.com/leaders.as...ter=&players=0