It would be tempting to use "justice" sad Aristotle's virtues most important to the city. But, justice is such an impossible subject to define and come to grips with. God knows, Plato and Socrates and their debating fellows attempted to create a meaning that would withstand all opposition. Instead, it seems that the idea of "friendship" is really the most important. Friendship is a very human characteristic. It is one thing over which ever human has some sort of control. Good friends make the difference between survival and living, one can easily surmise. Justice is usually an imposition. States and rulers create justice (or injustice, for that matter). There are no laws or ethical limitations to friendship.
Aristotle makes the choice even easier: "a'justice' and 'injustice' seem to be ambiguous termsa" (p. 376). He then spends nearly the entire Book V trying to go beyond what Plato (in "The Republic", among other works) has tried to establish as a virtue. In this book, it seems, Aristotle is more concerned about separating injustice from justice, the unjust man from the virtuous, just one, to really provide a student the opportunity to decide that justice is the virtue without which the city could not survive. There is, of course, some truth to that, assuming the city is an establishment, and not merely a community of individuals and separate citizens.
The rationale for choosing "friendship" as the virtue which is most important to life in the city seems top stem from Aristotle's belief that when measuring justice and injustice, "the significant of both consists in a relation to one's neighbor" (p. 377). He then clearly states in Book VIII that friendship "is a virtue or implies a virtue, and is besides most necessary with a view to living" (p. 406). While the Greek philosophers may not agree to jumping to such a quick conclusion, it would seem to the ordinary reader that man can live with one or more forms of inj...