Some forensic research could be done with the defective motors, to find out the reason for their disease. What's their current when they run free from 12V?
If the motors burn out, is it due to lack of cooling (air ventilation), load, voltage or limited duty cycle, misaligned axle?
A circuit diagram would be interesting, and motor and power supply ratings. Is the L294 still okay?
There is nothing like a "best" motor, no recommendation possible without further information about your project and requirements.