Objection 1. It would seem that justice does not stand foremost among all the moral virtues. Because it belongs to justice to render to each one what is his, whereas it belongs to liberality to give of one's own, and this is more virtuous. Therefore liberality is a greater virtue than justice.
Objection 2. Further, nothing is adorned by a less excellent thing than itself. Now magnanimity is the ornament both of justice and of all the virtues, according to Ethic. iv, 3. Therefore magnanimity is more excellent than justice.
Objection 3. Further, virtue is about that which is "difficult" and "good," as stated in Ethic. ii, 3. But fortitude is about more difficult things than justice is, since it is about dangers of death, according to Ethic. iii, 6. Therefore fortitude is more excellent than justice.
On the contrary, Tully says (De Offic. i, 7): "Justice is the most resplendent of the virtues, and gives its name to a good man."
I answer that, If we speak of legal justice, it is evident that it stands foremost among all the moral virtues, for as much as the common good transcends the individual good of one person. On this sense the Philosopher declares (Ethic. v, 1) that "the most excellent of the virtues would seem to be justice, and more glorious than either the evening or the morning star." But, even if we speak of particular justice, it excels the other moral virtues for two reasons. The first reason may be taken from the subject, because justice is in the more excellent part of the soul, viz. the rational appetite or will, whereas the other moral virtues are in the sensitive appetite, whereunto appertain the passions which are the matter of the other moral virtues. The second reason is taken from the object, because the other virtues are commendable in respect of the sole good of the virtuous person himself, whereas justice is praiseworthy in respect of the virtuous person being well disposed towards another, so that justice is somewhat the good of another person, as stated in Ethic. v, 1. Hence the Philosopher says (Rhet. i, 9): "The greatest virtues must needs be those which are most profitable to other persons, because virtue is a faculty of doing good to others. For this reason the greatest honors are accorded the brave and the just, since bravery is useful to others in warfare, and justice is useful to others both in warfare and in time of peace."
Reply to Objection 1. Although the liberal man gives of his own, yet he does so in so far as he takes into consideration the good of his own virtue, while the just man gives to another what is his, through consideration of the common good. Moreover justice is observed towards all, whereas liberality cannot extend to all. Again liberality which gives of a man's own is based on justice, whereby one renders to each man what is his.
Reply to Objection 2. When magnanimity is added to justice it increases the latter's goodness; and yet without justice it would not even be a virtue.
Reply to Objection 3. Although fortitude is about the most difficult things, it is not about the best, for it is only useful in warfare, whereas justice is useful both in war and in peace, as stated above.
Source: Summa Theologica (New Advent)