If I had to pick only one, then Tali. Honestly, she's one of the most sane person in my crew...aside from her hatred of the geth, which is understandable and somewhat rational.
Although, honestly, I love almost my whole crew...
Samara, Morinth, Thane, Zaeed, and Jacob I can live without though.
Samara is a Knight Templar, Morinth is a complete monster, Zaeed has no redeeming qualities, and finally Jacob is boring. Also, Jacob fits in a role that is better fulfilled by Garrus.
Dissatisfied by old job, quits and tries to do better elsewhere. Difference is Jacob joined a xenophobic human terrorist group, while Garrus started up a vigilante group on Omega.