Let's restrict the scope of possible answers to players Fabiano Caruana has faced since becoming a grandmaster, and has played at least four times. (No doubt there's some club player who has a 1-0 or 2-0 score against him from when he was a kid, and maybe there's some professional with similar results.)
There's a sense in which the answer should be Magnus Carlsen, as Carlsen is the world champion and long-time world #1. The question is referring to actual results, and while Carlsen's overall score against Caruana is good, of course, there's a better answer.