Historically or currently? Historically it is probably chris paul. Currentky probably Embiid. Combination, Lillard.
This isn't much of history. Wow, Charles, Karl Malone, Stockton, Clyde Drexler, Elgin Baylor, Iverson, Patrick Ewing, Reggie Miller, Nique, IMO are all above CP3.
As Kern said and as the initial post clearly implies it was players currently playing in the league, which none of those players are. Hence Chris Paul, who I believe has had the greatest career of the active guys to not have won.
For some support, the NBA's 75th anniversary team has the following players on it that haven't won a title: Harden, Lillard, Paul, and Westbrook. Of those, I believe Chris Paul has had the greatest overall career even though Harden and Westbrook have both won MVP's and Paul has not (though I had forgotten about Harden and he is close to Paul historically I think).
As I forgot Harden, I think I'd replace Lillard with him on my combination list. So greatest career: Paul. greatest player today: Embiid. greatest combination of both: Harden. I do think Lillard and Harden are close and Embiid has some company in Doncic and Tatum. This of course, as indicated in the opening post, that the Nuggets close out the Finals.