Has to be Kawhi. The only of Westbrook, Harden and Leonard trio who actually has interest in his team winning rather then bumping up his own statistics.
But with perennial GMOY and COY candidates, whereas Lebron has a team that basically can't win a game in his absence near the top of the league with a completely ineffectual coach and GM.
Spurs have a way better record than the Cavs have in a weaker conference and LeBron has two All Star teammates.
You're welcome to diminish the importance of a good organization, good coach, and good system, but I won't. The Spurs can (and do) win games without Kawhi. The Cavs can't (and don't) win without Lebron. Lebron is infinitely more valuable.