Kawhi is definitely the most capable, but in terms of guys that we could actually get, George is the man. LeBron is also a very lazy defender at times, so I'd be pretty comfortable saying PG is a better defender of wings than LeBron. Maybe even a better defender full stop.
Actually, if you want to get a guy who has the size, strength, agility, and athleticism to even attempt to offset Durant, look no further than Rudy Gay, provided that the significant injury doesn't take too much away from his physical gifts. He always plays his best against Durant, Giannis, Derozan, George, Lebron, and Melo, as well, and if that injury checks out, which, admittedly, is a big if, I'd love to get him.
I wonder if we traded for him, though, if we could get him to agree to not opt out of his deal. My only concern would be his handling at the hands of our "medical staff," as there's no way that he should play any back to backs next year, which would also free up more minutes for the young guys during the season, provided that he starts, that is, as he'd be a hell of a weapon coming in off the bench. What's not to like?
Edit: he just opted out, lol

. Ugh.