Check out Dave Holderread's book, of course--he has a breeds listing that shows mothering ability among other things. Very good stuff.
Off the top of my head, I second the muscovy suggestion--there is a feral one on our pond that raises several batches of babies every year without assistance and despite great odds against her. Among mallard-derivative breeds, I have read that welsh harlequins can be okay at it, as well as pekins. Of course, you can also get a broody chicken breed and use them to incubate ducklings. But personally, I'm thinking of getting a muscovy girl and letting her do my mothering each year.