There are some good suggestions in the answers Rottweiler Metallica master of pupies shirt given here and, sadly, there are also some unnecessarily condescending ones that, even more sadly, have been upvoted well beyond their worth. What you want is not naive. Indeed, it is perfectly reasonable and not an unusual set of attributes for many people who choose to get a dog. There are many breeds that could fit this bill but, of the ones that you mention, in my opinion the German Shepherd stands head and shoulders above the others. What some commenters seem to have taken issue with is your mention of the need to “be aggressive”. I think that this is simply a misunderstanding. Any dog that has to protect its owner from an attack will almost certainly have to behave aggressively to some degree.
However, that fact should not be confused with having Rottweiler Metallica master of pupies shirt to have a dog with an aggressive character, in order to have one that can protect you when necessary. I have trained many police dogs. Those dogs are trained to attack on command and make their target stop. As long as that target stops, the dog will let go of the target and sit or stand by until its handler arrives to take over. The dog will only persist in attack or attack again if the target resists or makes another attempt to get away. The handlers of these dogs can also have them break off their attack before they strike, if the target stops and surrenders. Although those police dogs are well capable of bringing down a large and fit man, they are also extemely stable and can be walked through crowds of people and petted by chldren and adults without any problem.