It sounds cliche, but I'd rather have an average-looking dog with brains over a great-looking dog without any day. The really intelligent ones can be too much for an inexperienced owner; I always warm up immediately to a dog who's got his owner trained! But the smarter they are, the more I like them. I suppose that's why I like dobies so much! Beautiful in body and mind, and sometimes too tough to handle, smarts-wise. I love the challenge!
Just strictly based on looks, though, I love the way a Canaan dog and a Borzoi moves, the fur on an Australian shepherd and a Chesapeake Bay retriever, the face of a pharaoh hound and Keeshond, the voice of a basenji, the muscles on a pittie, the posture of a standard poodle, a Shiba Inu, and a great Dane, and the smile of a Chow and a dobie. Come to think of it, those are all pretty brainy breeds.
So yes, I guess intelligence is the defining factor for me, followed closely by physical and mental health and stability, which would make my favorite breed your average MUTT!!! Smart, clever, funny, good sense of humor...mutts have it all!