Rottweilers, German Shepherds, Pit Bulls and Malamutes are the most aggressive breeds. They have to be socialized with people and other breeds. Early obedience training is a must.
All of these breeds, when socialized and trained, are some of the most loyal and loving animals. Even adopting an adult can be a joy. Observe their temperament. They should not be afraid of loud noises, sudden movements or aggressive with food.
If you are getting a puppy, go with a registered breeder. Avoid puppy mills. You need to see how the puppy is with its litter mates.
If you are adopting an adult dog, it should not be aggressive towards strangers.
And as always, spay or neuter.