Cheat List:
At the main menu, highlight Versus Mode. Then, in order, press and hold these buttons: L, B, R. Once holding them release them in this order to bring up cheat mode: B, R, L. Enter the following codes to unlock the corresponding monster:
All cities and monsters: 863768
All monsters: 209697
Black and white mode: 860475
Boxing Level: 440499
Extra Military Damage: 970432
Godzilla 2000: 637522
Regenerate health: 597378
Unlock Destroyah:
To unlock Destoroyah, beat Adventure mode with Godzilla 2000.
Unlock Gigan:
To unlock Gigan, beat Adventure mode with Aguirus.
Unlock Godzilla 2000:
To unlock Godzilla 2000, complete the Adventure mode with Godzilla 90