I'm going to be the only one who probably throws out his vote in favour of Robin. Robin adds something that Batman is missing; the human element. Robin, while painfully innocent and naiive, makes an excellent contrast to Batman. Where Batman's costume is dark, angular, and menacing, Robin's costume is bright, vibrant, and improvised. Whereas Batman lived his childhood scarred by the loss of his parents, Robin grew up in a happy family, bonding by literally putting his life in his family's hands as an acrobat. Robin plays an excellent foil to Batman, both a redeeming quality and a crippling weakness.
Robin, as seen by many, is nothing but a hindrance; he's ass in a fight, he's useless as a detective, and he often is the bait for many a Bat-trap. Not only that, but now Bruce Wayne's bank account has to feed another mouth, clothe another body, and equip another utility belt (not to mention how many vehicles and devices he has to de-Bat so Robin can handle them). Why, in the name of God would Bruce Wayne keep someone so useless around. Simple, in Robin he sees something he longs to be. He sees the childhood that Robin had, and longs to know that warmth. He sees Robin like a brother and a son, and so that strong emotional attachment gives Batman more of a human quality (arguably more than any other character besides Alfred). Batman could always leave Robin in the clutches of the Joker, abandoning him to a cruel (if not inventive) death, accompanied by cackling laughter. However, he won't, because in Robin he sees himself, or at least a Bruce Wayne that could have been.
