Saxony! The Saxony and Silver Appleyard are both large breeds that lay well. Most domestic ducks will lay fairly well, the best will be Runners, 300 hybrids, and Khaki Campbells. But the Welsh Harlequins, Appleyards, Swedish, Saxony, Rouen and some strains of Pekin can't be called bad layers. There's others too. Some Pekin have been bred more for meat, others for dual purpose.
My Saxony hens haven't skipped a beat until one went broody. The other still lays EVERY single day, and has since February when they turned 6 months old. My other hens that I no longer have laid right on through winter without added light or heat.
Khaki Campbells. Welsh Harlequins, which are a derivative of the Khaki Campbell. Indian Runners. Metzer's Golden 300 or Metzer's White Layers (these are hybrids, so replacements must be ordered, not bred - not an issue for pet ducks, an issue if you homestead or farm)
I chose Welsh Harlequin because they still have a broody instinct, so they could hatch out replacements. When they are broody, egg production goes way down . . . but mine laid all the way through the winter, when my chickens quit!