I have a German Shepherd, she is SOOOOOOOO sweet--she loves to play and is always exited. She's a good natured dog, but when she senses something's not right, she'll come to full alert, ready to move in on whatever it may be-she's and exellent guard dog.
I love a Rottweiler for alot of the same reasons: they're intelligent, obiedient, playful, companionable, beautiful, and a very large, wanderful guard dog!
I would recomend a female over a male because females are less aggresive(towards you), but not to strangers, they are also less likely to wander off!
Our smaller dog--Flap--is a mix between a Chauahuah, Pikenese, and a few other small breeds, he's an exellent watch dog too.
Lara