Tali was incredibly good for the actual solid definite character develop in game rather than giving most others an excuse to talk to you about their character development.
EDIT: not to say none of the others do it at all, i just felt Tali's developed the most, directly responding and forever changed from events actually taking place, rather than post event reflection.
Also the Samara/Morinth mission was good due to the confrontation of the ideas of both "the sins of the father" AND the questioning of whether one can truly call Morinth a villain or not, as it seems her birth is what gave her the gift/curse as a result of her mother. The questioning of whether Morinth is justified in enacting the punishment of her mother (through the killing of others) for you know, not restraining her own desires and demanding that Morinth must do what Samara could not.