What a fun question!  Well, obviously this is open to interpretation, but here is my opinion of the "best" character trait for each of the main characters:

Romeo: Passion (as evidenced by his every word and action and culminating in his death by poison)

Mercutio: Bawdy Humor (as evidenced by his efforts to break Romeo out of his "funk" with such stories as Queen Mab)

Juliet: Coy Intelligence (as evidenced by her flirtation with Romeo upon first meeting him and her innuendo in referring to men's body parts)

Tybalt: Abrasiveness (as evidenced by his desire to fight in ever scene he inhabits)

Nurse: Good-Natured Humor (as evidenced by her exchange with Juliet after returning with news from Romeo)

Paris: Chivalry (as evidenced by Paris' desire to lie in death near to Juliet)

Friar Laurence: Loyalty (as evidenced by his great lengths to both marry the young couple and desperate tries to help them live happy lives)