And again this is easy... because there is actually only one movie dog who ever had my heart, the one I still want to this day. That's Dog from from my third favorite movie of all time, Big Jake.
He's a collie (apparently from the Lassie line), but he doesn't have the traditional coloring... or rather he does, but they dyed his coat dark for the movie, which is half of what made him look so cool. Sadly, because it's dyed, it also means I will never ever be able to own a collie that looks like that. Sniff.
Dog was the epitome of what a dog should be to me. Beautiful, big, playful, tireless, quiet and patient, growls a soft warning when anyone approaches, attacks or does other helpful things on command, and is faithful to the end. I adore Dog. There have been other faithful dog companions in films, but none of them affected me quite the way Dog did. Because of Dog, my favorite canines will always be big, dark, and with upright ears. That's what a dog should look like.
One of these days I still might get a collie, though I wouldn't name him Dog, LOL! Most likely, I'd name him Batjac or Jake, but, of course, names depend on personalities, so who knows. My own dog right now is a grey/white Siberian Husky named Maximus. He fulfills a few of those traits I love in Dog, but mostly he's just big, beautiful, and playful (and gets along with the cats). No quiet warning growl, alas. He's got a loud howling bark he uses if anyone's comes around. :-D
(Max... click pic to make bigger)