I personally like flash most, but I put my vote for superman because of the way the question is worded. Superman is the "greatest" DC hero. Is he the best hero comic wise? No, I think Batman comics are probably the best. Is he my favorite? Obviously not since I already said my favorite is flash.
So why did I vote for him? Because he is the example for every other hero. He's the shining pinnacle of the American justice system and is the kind of hero that I would most want defending me and my city. He follows the same rules as the police, can be everywhere at once, will always put others above himself, etc. etc.
Superman gets my vote.