1. Sally Sparrow (Blink)
Only in one episode and sharing the screen with the Doctor for less than five minutes, Sally Sparrow nonetheless left her mark on the show. She was intuitive, brave, and had a keen knack for problem-solving that the Doctor always seems to prize in his companions. Sally also fits the modern template for the companion role -- so really there's no reason why the Doctor wouldn't let her tag along for a bit. Sadly, with Carey Mulligan's rising star in Hollywood and the Weeping Angels just this side of "done to death", we're unlikely to see a reprisal of the character we all fell in love with. But Moffat, if you're reading this, I would totally settle for Inspector Billy Shipton as the smoothest companion we've ever had. I'm not picky, me.