Definitly Cloud, Tifa, and Vincent. Cloud and Tifa had their lives turned upside down by Sephiroth. Cloud lost his mom, if I remember right... he was basically tortured, and his entire psyche was torn up because of what Sephiroth did. Tifa lost her dad (which is even worse after having lost her mom), was nearly murdered, and then basically lost Cloud, even though she manages to get the real him back eventually.

Vincent wasnt effected by anything Sephiroth did, really, but the attonement thing that has been mentioned is very true. He's still intertwined with Seph, unlike the rest of the group.