Looking at all the Bball Ref numbers, IMO RWB is the only choice. Plus minus way over anyone else. Higher assist% than Harden. I was not even a big fan before this season, but this effort is incredible. Doesn't mean he is better than Leonard or Lebron obviously, or that he could ever lead a team to a title (neither could Harden probably). But the body of work is overwhelming this season.
I was not a Harden fan either, but he has really impressed me as well. Meanwhile, LBJ does the stuff that wins titles, like 59 eFG and 27/8/9. Best player in the league every year.