you didn't specify size. does that matter? An Italian greyhound would be a good choice if you want a smaller version of some of the breeds listed. Most of the other breeds I could think of someone else already posted, but I have one more that's idea of a dog that fits your description but it's a little bigger than an Italian greyhound: Russian wolfhound aka borzoi.

"The Borzoi is a quiet but athletic and independent dog. Most Borzoi are almost silent, barking only very rarely." - Wikipedia

apparently they aren't that intelligent and easy to train, though.
I read somewhere that black russian terriers are very intelligent and obedient and they're pretty athletic and strong. I heard BRTs also make good guard dogs. I've never seen one, but from the sound of them I like them. A little rarer and more expensive breed, though.

If you want any more ideas, I suggest you do a search on the sighthound category of dogs (dogs like greyhounds, irish wolfhounds, borsoi, itallian greyhound, etc). they seem to match your description fairly well, especially if you want a thin dog, but of course there might be other dogs besides sighthounds that might interest you.