I guess Lebron? It's tricky, because it depends on the rest of the hypothetical universe. If all the other players are free agents, then of course LeBron, because I'm going to get two other max free agents of my choice, and I can fill out the roster with ring-chasing vets costing on the cheap.
If it's this past summer, do I get KD and Horford to join LeBron? I probably could, so yes.
If it's next summer, however, I probably can't afford LeBron and KD and a third star, because KD is now on the 10-year salary level. Chris Paul gets in the way for the same reason. If I can't construct my win now team around LeBron, I'd go Towns.