I would say Cayugas or Pekins(Jumbo or otherwise), but that's just from my experience. Individual ducks within a breed vary greatly. I've only had two Cayugas in my life, but both of them were really tame and friendly. I know they are supposed to be a calm, quiet breed, and I have found this to be true so far.
The Pekins I now have, which are Jumbos, I have found to be really friendly. Not quite as tame as my Cayugas, but still very sociable. I have also had two ordinary Pekins in the past who were not quite as friendly, but I am willing to say that that was just them and not necessarily anything to do with the Pekin breed in general. I have heard that Pekins are supposed to be good pets.
I have heard that Welsh Harlequins make good pets as well, but I only have one, and she is not as friendly as some of my other ducks, though not unfriendly by any means. So I don't have enough experience with them to say. But really, almost any breed can make good pets. It just depends on the specific duck. But there are those breeds that shine as far as friendliness and sociability with humans goes, and I would say Cayugas are at or near the top.
I know lots of people on this forum have Muscovies, and love them. I've never had any myself, but I hear they are excellent pets, along with providing good meat and eggs.
Hope this helps!