Here are the breeds everyone is going to tell you: Khaki campbells, Golden 300s, Welsh Harlequins, and Indian Runners.
And they will be (sort of) right.
But I learned something new this year directly from Dave Holderread (world-respected waterfowl expert): more important than breed is the line.
The reason for that is that anybody can buy an Indian Runner and start hatching babies. But if the breeding stock in each generation are not carefully selected for laying ability, they will quickly lose their edge, and before long you have ducks that look like Indian Runners but lay like Pekins.
The moral of the story is: buy a good laying breed from a reputable breeder who carefully selects for laying ability.
Holderread Waterfowl Farm is an excellent choice--any of the high-laying breeds from his stock will be good choices all around--hard to beat all the way around. You can also choose a local breeder if you can find one that selects carefully for ability. The production hatcheries--McMurray, Metzer, Ideal, etc.--will produce good egg qualities with pet-quality looks (speaking very generally). You might find a good breeder on these boards too-lots of excellent options here--just be sure to ask whether they select for laying ability.
I'll be frank--I don't. My birds are all second or third generation from either Holderread or McMurray, and I don't have the capability to select for egg laying at this point. So my birds are selected for looks primarily. I am up front about that--they still have good laying ability, because I started with good stock, but the laying ability will probably taper off after a few generations if not carefully monitored. I do plan to add laying ability into my plan, but I'm not there yet.
Just check with whoever you buy from, and look for signs that they're giving you straight answers. My birds *do* produce good quantities of eggs, but birds bought directly from Holderread would probably be even better if egg production is your primary concern. That's the kind of answer you want from your breeder, and then you can make an informed decision.