About the topic, I don't know about "highest", but he's definitely very good at reading the D and making plays, avoiding mistakes (most of the time) and playing an efficient game. Of course he's also in a great position to do that given his stacked team, which fits his game as well as any surrounding cast has ever fit a #1 option.
Still, I would consider LeBron among the "smartest" on-court players, in terms of his reading and reacting instincts.
You could also think of "basketball IQ" in terms of a player's depth of knowledge about the game. In that sense, I don't think LeBron is so high up, meaning, I don't know how good of a coach he'd be, because it's completely different. For example, despite him being my favorite player, I wouldn't put Kobe among the top guys in terms of on-court "smart" and efficient decisions.. He's not as careful as some of those guys (probably a good thing in some ways), he does tend to take wild shots from time to time (though again, it's also about the guys around him and the position he's in) and he ends up being a less efficient player. But in the sense of basketball knowledge
, which is another way to think about bball IQ, I think he's definitely up there as a player; I feel would be a great Xs and Os coach, because he seems to really understand the intricacies of the game (especially when you hear him talk and breakdown the game).
So there are different ways to view this "bball iq" thing, and also we can't forget that much of what we perceive of it has to do not just with the player himself but with the position he's in, in terms of system and personnel.