My favourite is charity. The following is a train of thought that probably leads nowhere. I recommend changing at the next station for a faster train; it'll still get you nowhere, but at least it will get you there quickly.
It seems the premise of most people's response is that you have to pick your favourite and then imagine that you have this favourite in abundance and none of the others. That seems like a silly premise. If that were the case, then my favourite food would be bread, my favourite drink would be water, and my favourite hobby would be breathing. Rather, my favourite food is the one I enjoy eating the most, my favourite drink is the one I enjoy drinking the most, and my favourite hobby is the one I enjoy doing the most. So my favourite virtue is the one that I enjoy the most, and that is clearly charity. I enjoy charity much more than I enjoy those other dull things like prudence and temperance.
But perhaps that's a reductionist take on the question. Perhaps, by "what is your favourite?", the OP means something else. Afterall, it's a strange question to ask of virtues. You don't live virtuously because you enjoy them, you live virtuously because it is the right thing to do. So perhaps what the OP is asking is which virtue is the most virtuous? This seems like a difficult question to answer. How do we go about answering it? What does "virtuous" mean? Is "virtuous" even ordinal (things can be more virtuous than other things), or is it simply binary (i.e. a thing is either virtuous or it is not, and one can't order them)?
On the face of it, all of those things are virtuous; that is the premise of the question. I can stop there, and say that all of them are virtuous, and asking which is more virtuous than the others is nonsensical, akin to asking which tree in a forest is the most tree-like.
But that seems unsatisfactory. After all, we have a special word, "virtuous", which seems to imply that it is not binary, and that things can be "more virtuous" or "less virtuous" than other things. Indeed, we can prove this straightforwardly. Take charity. If I give £10 to charity, then this is less charitable than if I give £100 to charity. The more I give to charity, the more charitable I am, the more virtuous I am. I can be more or less prudent, or more or less faithful, or more or less just, and so on. So virtue can be judged ordinally.
But that isn't enough to answer the question. Perhaps each virtue represents a different axes of virtue, independent from one another, and not comparable to one another. Perhaps, while you can order virtue along each axis, you can't order each axis with respect to the other axes. Metatron would interject at this point and say, "but they're not independent! You can derive the other axes from 1 single axis!" Well, I agree with Borachio here: I think Metatron is cherry-picking his definitions. I think you can couch each other virtue in terms of prudence. But I also think you can couch prudence in terms of all the others. I agree that prudence implies rationality, and I agree that those other things are all prudent. But I don't agree that prudence implies those other things. It might do, or it might not. A great many things are prudent, but not all of them are charitable, temperant, just, etc etc. A great many things are charitable, but not prudent. A great many things are just, but not charitable. And so on. I would argue that each axis is independent, because none of them logically imply the others.
But even if our axes are independent, it doesn't mean that our "virtue function" weights each axis equally. It might be that the "coefficient of prudence" is greater than the "coefficient of justice" or the "coefficient of charity". Fine, I agree: some things might have a greater coefficient than others. But, to me, that isn't necessarily important. Let's say that justice, which is currently winning in the poll, has the highest coefficient in our virtue function. So we would say that justice is the most important virtue. Or would we? Imagine if the entire world was filled with people with an incredibly strong sense of justice, to the point where literally everybody has a high score on the justice axis of virtue. Would justice be important in such a world? Perhaps, in a world where there is plenty of justice, it is more important to exhibit the other virtues instead, even if they don't contribute so much to the virtue function?
So what does the world need more of? What does this country need more of? What do I need more of? Prudence? Maybe. Justice? Perhaps. Charity? I think so. I think that the world needs more charitable people, people who are kinder and more thoughtful. And I think that we all, as individuals, could stand to be a little more charitable. I think we've spent a lot of time over the past 400 years or so putting in place institutions, structures, checks and balances to ensure that we live in a just, prudent society. And over the past 100 years or so, we've worked hard to make this country a more charitable place, in some ways compensating for our individual lack of charity. But whereas those structures for justice and prudence have been getting stronger and stronger, our welfare state is being rolled back every year, our taxes are becoming more regressive every year, our society is getting more unequal, not less. This is the crux, the point where justice, prudence and charity converge. And right now, justice and prudence are winning. It is prudent, they say, to cut spending on welfare and pay down the national debt. It is just, they say, to cut spending on welfare at a time when working families are being squeezed. It is just and prudent, they say, to be less charitable at times like these.
But I say that at times like these, we need to be more charitable, not less. And if the government won't do it, then it's up to individuals to pick up the slack. The most important virtue, the virtue this world needs more of, is not prudence or justice. We seem to have that in spades. What we need is charity. We don't do nearly enough of that.