W.C. Fields advised actors never to work with animals or children, and for good reason: When a dog is onscreen, it's hard to watch anything else. Canines like Marley, Bruiser, Sam from I Am Legend are the best friends their owners ever had. All of them are brimming with personality, most are skilled at protecting, but only one of these loyal hounds can claim to be man's (or a woman's) best friend -- which one of them will it be?