Get a shelter mutt. My dog is a shelter mutt and he is the most awesome and loyal dog ever. I live in an apartment, too and know many others who live in them. Avoid getting a large dog. Many apartments have a weight limit on dogs, and for good reason. They tend to make a lot of noise from romping around and the neighbors below complain, and large dogs need a hell of a lot of exercise that is not compatible with apartment life. I personally find small dogs highly annoying, so I got a medium sized lab mix at the shelter. His name is Aries. I'd say for minimal grooming also get a dog with short or medium length fur, but my dog has long fur and I only need to brush him out a couple times a year.
Thank you everyone for all of your input so far. I just talked to my BF about possibly adopting a Greyhound- there's a dog track near here that adopts out retired racers- and he wasn't opposed to it. They also apparently allow you to use their track for running your sighthounds.
Just to clarify, I'm looking for a dog with at least above-average energy levels, because I'm planning to have them as a workout partner as well as a watchdog. I don't want a dog that is overly sensitive or will continue to bark long after the 'threat' is gone, just one that will let us know when someone's at the door or something doesn't seem 'right' to them.
Depending on what region you're living in, you might want to consider getting a cur. They're more common among hunters in the Appalacians and the South. Curs are good guard dogs, very territorial, but they can be taught to be very calm and social when away from "home."
They're short-haired dogs. The black-mouth yellow curs often get mistaken for yellow labs, so there would be little worry about the dog being stolen or anything like that.
The New Jersey wildlife department employs yellow curs to help run off nuisance bears from residential/public areas, so you know these dogs are tough. They're not intimidated easily by size or injuries.
Personally if I were getting an apartment guard dog, I'd either get a chow-chow or Eurasier. Nowhere near as much grooming as you'd think, they'll appreciate a brush or two a week but other than that the fur takes care of itself. No smell (which certainly can't be said for the german shepherd... in a house it might not be a problem but an apartment gets smelly very fast), no fuss, moderate activity level, perfect for apartment living. And both breeds have been known to scare off anything from burglars to bears. And also they'd be able to put up with our Swedish winters... But if you have a good feel for Doberman, I think that's the breed you should get. They're wonderful dogs, big barks and big hearts.
Define a "guard dog" as if you are wanting a dog that will bark at intruders, you have a lot of options with tricks like barking when someone shushes them or just naturally barking at people, especially for any that are a bit pack minded. But do remember that almost every dog can be bribed with a piece of chicken in your fridge and those that can't be bribed are going to be extremely aggressive to strangers.
Remember that you are also at an apartment complex and it is aggreviating to neighbors if you have a dog that is constantly barking at people running by or gets extremely aggressive at any stranger who so much as gets close to the house. Frankly, I would almost argue that you don't need a guard dog but instead need to practice home safety.
I would check your local shelter for an already adult dog if you are serious about wanting it for guard dog duty. It is the bark that deters intruders, so you want an adult dog bark, a puppy really does not do much good.
Not to mention that an adult dog is already trained.
Personally, think German Shepherds and Dobermans are too big for apartments. Kind of overkill. They'd be slamming into the walls and knocking things off tables. Many apartments that allow dogs have a size limit. Also, in my opinion, GSD's take a lot of grooming. At least too much for my taste. Ok, I like dogs with zero-maintainance coats.
There are many dogs that will bark and raise a ruckus at any sign of danger, I think this is more important than one that can physically stop someone who's breaking in. A barking dog can dissuade many would-be robbers before they even get in the house. If I were you I'd be looking at dogs between 30 or 40 pounds. One idea would be getting a mutt that's a cross of one of the breeds you're after, but without the size. There is a STANDARD pinscher that is way smaller than a Doberman but IMO has many of the same traits. But anyway, I think any kind of active, alert, intelligent dog would be good for your needs.
Well, that's my opinion, there's no one right answer. If you feel you really need a 70 lb dog that can really fight with a burglar, you won't be happy with a smaller one. You're obviously familiar with what having one of those dogs entails, so I won't elaborate.
I kind of lucked out, I have 40 lb mutt with really short legs and a massive head. We even think he's part rottie, although obviously we're not sure. That's what they should breed, a mini rottie. What fun!