Nutrena Naturewise "All Flock" pellets. Nice small pellets, even ducklings eat them fine at about 2, 3 weeks of age. Very little waste. The chickens, ducks and geese even go after any spilled pellets.
Right now I am feeding laying mash, only because it is convenient because that is what my other birds eat. When I can afford to buy the ducks food just for them I get pellets. My ducks get to be let out, at least for a while every day so they get some free ranging anyway. I am rather surprised that they stay in the back yard, when they could move just a little further off to the side yard and get lots of nice green grass and weeds.