Which solution has the largest concentration of ions?

It depends on the solution. For example, a 1M solution of MgCl2 has a greater concentration of ions than a 1M solution of NaCl because MgCl2 breaks apart into 3 ions while NaCl only breaks into 2 ions. On the other hand, a 12M solution of NaCl has a greater concentration of ions than a 1M solution of MgCl2 because the NaCl solution is simply so much more concentrated with NaCl than the MgCl2 solution is with MgCl2.