Well I guess I would be the first to respond to this post.
What is a virtue? It can be defined as that which is morally good, therefore courage that exhibits that added incentive of being morally right is considered a virtue.
There were many characters in the novels that showed their bravery, but were in no way virtuous. Bellatrix and Voldermort to name the most prominant.
They can be viewed as brave because they stood for what they believed in the face of strong opposition. However their fight was not one of virtue and therefore their courage, though very real was not a virtue.
To me, the ultimate character that exhibits the the slogan "Courage is a virtue" and who in turn possess that virtue is Dobby. Dobby understood from early on what the moral right was and he stood against teh Malfoys, his masters, in his attempt to stand for that right. That is courage. When one stands against those who oppress, for a moral purpose that is inherently good, then he exhibits courage of the highest order. He understands the severe consequences that he would endure, yet he still acts. These points relate to his role in COS.
As an ultimate act of courage though is shown n DH, where he places danger second to that of loyalty and friendship and thinks not of himself but of those he can help, he makes the ultimate sacrifice for the "greater good".
As you stated in your post some ppl consider Dumbledore to be the bravest or Harry or Sirius, but I think that that honour should fall on Dobby, whose courage is a virtue!