Dutch Hookbills are the calmest, most mellow ducks I keep. My Indian Runners are a close second - not at all mellow, but very bold and inquisitive.
My Campbells are all a little more suspicious of people, even though they were raised along with the Hookbills and Runners and received the same amount of handling.
Really though, friendliness depends on the temperament of the individual bird. You could have a Campbell who's a friendly, cuddle-loving duck, or a Indian Runner who's aloof and regards you as an evil creature that should be avoided at all costs! Handling them frequently as ducklings is very important too - NO duck, regardless of the breed, is going to be "friendly" is you don't handle them and get them used to people while they're little.