Big dogs do very well, but I can't recommend getting a dog as a security device if you don't have any other desire to own a dog.

Buy good replacement cost insurance for you posessions. After appropriate training and practice, arm yourself to protect from physical harm. There are things you'll need to be intimately aware of being armed in an apartment situation. Safe lanes of fire, what's behind every wall that may be in your line of fire, etc. Don't ignore that part of it.