I don't think it's reasonable for me to pick just one. I think that if a person had an unlimited supply of just one of those virtues, but lacked all others, then they would be a horribly incomplete person. So I can't consider one to be "greatest" if it is not complete as itself.
My list of most important virtues, looking through that article:
-Altruism (which would include compassion as a subset, and usually love)
-Awareness
-Courage
-Creativity
-Diligence
-Prudence
-Reason
-Strength
If I absolutely had to pick just one, I'd go with prudence. But that's kind of cheating since prudence by definition includes a bunch of other virtues. Still, I think it lacks completeness and must have other virtues to go along with it.
Interesting choice. Why did you choose this one?
I put creativity on my list so I'll give an answer:
Creativity, the ability to create and bring into existence new concepts or things, is the very core of what many people would say makes life interesting or what defines us in our human condition. Out of that list of those 8 virtues I find the most important, 5 of them are easily held by many animals. A dog can be altruistic, aware, courageous, diligent, and strong, but it's difficult to say a dog is highly creative (cunning or intelligent maybe), prudent, or has the ability to reason anywhere near the level of a human.
-Lyn