Batman. Batman has multiple contingency plans to take down the whole JLA, just in case they go crazy or something. In a recent run, Ra's Al Ghul got a hold of just one of these plans, and it totally worked. They kicked him out of the JLA for being too dangerous. Not just Superman. Not just Wonder Woman. Not just Flash or Green Lantern or Aquaman. ALL of them.
In a similar vein, Superman gave Batman a Kryptonite ring to take him down, because Batman was the only person he fully trusted to pull it off.
Plus, I'm not convinced any of the others would be able to handle the Joker when he's operating at full steam. Well, at least not without losing their sanity or morality.