Depends entirely on what you want them for.
But just off the top of my head? I'd say magpie. Or ancona. Or welsh harlequin. Or call. Or runner. Or.... wait....
Okay, let's go with magpie for a minute:
1. They're gorgeous. Stunning black, blue, or chocolate on a black background. Classic duck shape.
2. They're great layers. My Holderread magpie stock started laying right at 5 months and are giving me an egg a day even in November.
3. They're gorgeous. Everyone who comes to my backyard asks me what they are and remarks on their stunning looks.
4. They're calm. Very docile ducks.
5. They're gorgeous. But I might have mentioned that already.
6. They're good (but not great) foragers, providing a lot of their nutrition for free (but you still have to offer a good diet)
7. They're gorgeous.
8. They're gorgeous.
9. They're very unusual--quite a conversation piece.
10. Oh yeah, and they're gorgeous.
Down sides: they're supposedly not great mothers (don't know from experience--mine are still young); although large enough to eat, they'd still make only a small meal for a family; they might be hard to find locally, but Holderread Waterfowl has gorgeous stock available at reasonable prices if you order in advance
But really, there are so many wonderful breeds, and it all depends on what you like.