I would of course buy an M3 P-38 or Corsair. These guys have proven that they know what they're doing with the MiG-21 despite some remaining niggles.
A MiG-23 is becoming ever more necessary for DCS though. As a foil to the F-5E and future F-4 modules the MG-21 doesn't really cut it in that environment.
Just to say that if it were possible for M3 to begin work on a MiG-23 it would have a very positive effect on the multiplayer scene as it stands, more so, I would argue than a P-38 or Corsair.