Simmons has the highest ceiling of the group, as he is a good jumper away from being in the discussion for the best player in the league. However, between his public attitude regarding the topic and the results on the court so far, he’s not anywhere near making that a reality.
If he doesn’t develop a jumper, he still has the ability to be the best of the three, but it becomes more likely that Luka or Tatum pass him. I think Tatum has the edge in this scenario, as he has the length and athleticism to be a transcendent defensive force.