I'm judging the gold dragon a bit too harshly, I think. I mean at least it can breathe fire, which is one of the main criteria for being a dragon in the first place (if it doesn't fly, hoard gold, and breathe fire, it's just a weird lizard.) And I'm being unfair to the black and blue dragons too...I mean, acid will "burn" flesh, and lightning can ignite combustible materials, which is almost like breathing fire. So I'll concede that black and blue dragons are almost real dragons, and should be more toward the front of my list. ;-)
So after further consideration:
Red or Gold >> Blue and Black > any other chromatic > any other metallic > dragon turtle.