In general, the bigger the dog, the bigger the bark. The current holder of the Guinness Book of World Records title is a German shepherd dog from London named Daz, whose mighty mouth-off measured 108 decibels, somewhere between a diesel truck at 30 feet and a chainsaw at 3 feet.
For sheer volume of bark, you can't beat a Great Dane; however, you may get this intimidating sound from behind you, as Danes are not known for manning the front lines. Giant breeds, such as the bullmastiff and the Great Pyrenees, tend to have a big bark, but don't use it too often.
For volume of sound over time, look to those bred to guard flock and farm. This includes the various herding breeds -- shepherds, collies, komondors and maremmas. Doberman pinschers and Rottweilers add speed and muscle to their ferocious barking. Sled dogs such as the husky, the malamute and the American Eskimo will raise collective hell over an intruding polar bear (or an unreasonable facsimile thereof), and hounds such as the beagle and the basset will bell and bugle to alert all and sundry.
For sheer irritation of loud and persistent racket, bet on the small fry. Miniature and toy poodles, Pomeranians, Chihuahuas and that tiniest of terriers, the Dandie Dinmont, are champion barkers, but the biggest bark from a small package comes from the miniature dachshund. This is disconcerting to a burglar, because the sound barrage comes from something he can't see because it's under the bed or the coffee table. Once these guys start, they don't usually stop until their pack leader calls them off.
Whatever the breed, any dog that barks too loudly or too much can become more than a nuisance. Any prolonged sound above 85 decibels can damage human hearing, and even a single woof of 120 decibels would be painful. Find out why your dog is barking and do what you can to alleviate the problem, be it loneliness, boredom or overstimulation, or your best friend may involve you in social unpleasantness or even legal difficulties. On the other hand, his mouth might just save your life.