If you want a guard dog its very hard just to genralise and mention various breed IMO.
These dogs mentioned above in general are a far cry from being bred for any kind of purpose, its more about money and exaggeration. Before the breed you need to find a good breeder that takes the original purpose and temperament into account. Not some idiot just breeding the biggest baddest loudest dog on the block with a huge chain thrown in for effect.
Take the Am bull i recently judged at an American Bulldog show in the puppy class and was horrified by the terrible nerve of the dogs, even at that