#1 don't ever for get whistling ducks, mallards will respond to them.
For just mallards soft sounding quacks and natural clucks has been the best for me. Clucks? Yes, Hen malllards cluck listen to the ones that are feeding or in a local park you will be amazed at their sounds. Also try drake grunt that will bring them in nice and close. Mallard drake grunts can be made on most whistling duck calls, making them able to call many species of ducks all in one