For stability,
Phase margin > 0. Good value: 45 - 60 deg
Gain margin > 0. Good value: 5 - 10 dBs
Phase margin: Measured as the difference between the phase angle and -180° when the magnitude of the open loop system crosses 0dBs.
Gain margin: Indicates how much the gain at the phase crossover frequency can be increased before the system is driven to instability