Well i don't want to promote MY breed....but i have to say Leonbergers are extremely calm from day one,
They are a giant breed from the companionship group and even at 8 months my boy is like a rug in the house and bomb proof with humans, he knows when a person is old and frail or between how boys like to play and girls.
He really amazes everyone he meets, he loves other dogs and is full of energy when he plays with his friends. But when its home time he is calm on leash and a joy to live with.
BUT even though leonbergers are renown for this personality trait, i have always crate trained him, worked on leash manners and take him to obedience saturday and sunday.
I always factored in lots of training and socializing from the day i brought him home. ANd never let puppy cuteness get away with something i wouldnt want a 80 kilo adult to do.
He is and adolescent now and he doesnt like his commands but he is still an angel inside, its just off leash he is flexing his adolescent muscles at the moment.
But a leonberger is not a dog to adopt without total commitment, they bound so strongly with their families they suffer a broken heart when rehomed.
Here is a (not very good picture) of my boy Rox
Running with his Boxer buddy
Rox with my son