"Optimum" is different for everyone. For flying saucers, optimum is low and slow and smokey. Speed and altitude are not part of the equation.
For the 12", Original flying saucer, the optimum motor is the Pro38, 1 grain, G79SS. If properly constructed, it will take a Pro38, 3 grain motor with no problem. It will also fly well on Aerotech H73J and H123W RMS 38/240 motors and Ellis Mountain H48-P single use. I don't recommend any motor with a fast burning propellant such as Aerotech Blue Thunder. In general, it is best to fly saucers and other high drag designs with long burn (>2 seconds) motors.
The optimum motor for the 10.25 inch original flying saucer is the Ellis Mountain G35. It will tolerate an Aerotech G80T well enough, but the G80 won't make it fly any higher than the EM G35.
I would not recommend the H238T or any H motor and above in the 10.25", 29mm Original flying saucer. The airframe is not designed for the stresses involved.