Ayane hands down for me. I'm looking forward to the long road of getting her down. Her attitude and demeanor match her fighting style perfectly all the way down to the abrasiveness. Kasumi is the same way.
Helena as the different stages of her mother's death is kinda illustrated in each game (anger with DOA2, bargaining with DOA3, depression with DOA4, acceptation with DOA5). Wonder what the new game will bring.
Jann Lee, because he's the kid from the slums who watched Kung fu movies and trained to be able to help others instead of just watching, the true hero.