If you're just adding LeBron to a roster then I think any team he goes to would make the playoffs in either conference.
However, if you're replacing the current best player on each team WITH LeBron, then it gets a little dicier. I still think he takes most teams to the playoffs, but I'm not sure if you replace Demarcus Cousins with LeBron James that the Kings win the 50 games it'll take to make the playoffs in the West. Or if you take Derrick Favors off of the Jazz and put LeBron on there or take Kevin Love off the T'Wolves and put LeBron on there, those teams are still

And even if he got those teams to the playoffs he'd be

by April.