The DK seems a good choice, most of all because they start out at lvl 55. According to blue posts, all the tanking classes will be viable heroic tanks with the right spec/gear. Since we don't really know how the DK will end up, it's difficult to say, however what makes the paladin shine in tanking for multiboxers is that it doesn't require a lot of micromanagement. I don't know whether this is true for the DK or not, but I'm guessing the answer to the question is closely related to whether it'll require a lot of micromanagement or not.