I believe Cloud finnalying being able to kill Sephiroth is right,because no one else had such a living hell as he did.He was a puppet of Sephiroth and Jenova,and to finnaly beat the man that had used him and destroyed him so much was truly right and perfect.

But,if not Cloud,then Zack.Zack for the resons because he was going to be one of the clones for Sepgiroth.Perhaps Zack and Cloud should have done it together,but it wasnt posable