I am using a Darlington array http://arduino.cc/en/Reference/StepperUnipolarCircuit to drive a unipolar stepper. Would you please advise what diode I should use at com? Thank you.
Anything really, I would go for a 1N4001 to 1N4007, the last digit just indicates the reverse breakdown voltage which is not relevant in this case.
The ULN2003 has built-in diodes. You must connect one pin of the chip to your motor's positive power supply, so that the diodes work.