I don't know how much money you want to spend, but, St. Bernards are great dogs! They are usually mellow and dog friendly. They can be trained to be a guard dog, but it can be hard, they love people!
Another dog would be a Bernease Mountain dog, they are a lot like a St. Bernard. I hear that they are good dogs, there is breeder in my town that I could get ahold of to ask questions to if you'd like!
Good Luck!
Steph