Overall, I'd say Cloud. The layers of experience, torture, and deceit piled up on him pretty fiercely. He lost his mind and identity, only to be reminded of the truth in the harshest way possible. He basically rebuilt his existence, even as he came to terms with overwhelming loss and the similar burden of trying to save the planet from a disaster he unwittingly had a hand in. Incidentally, the way Cloud confronted and overcame his weakness is what makes him my favourite 'hero' of the series.