I think the hardest dog I've run across to train was a chesapeake bay retriever. Extremely hard headed!! Once you get them trained - they NEVER forget. Great dogs, but they do require patience.

They can also be the hardest to control in certain situations. They are very territorial about the person they pick as theirs. If they sense a threat to their people - yikes - get out of the way. At one time, we had three (weighing over 300 lbs total) and they were soooo mellow until they sensed a threat then watch out.