I think Zack is stronger. He survived 3 fights with 1st class Soldiers, 2 Mako showers, and fought off an army long enough to kick their ass to save Cloud's. Cloud survived a stabbing, Mako poisoning, and an identity crisis. He's also stronger emotionally. Cloud saved the world and what, fell into a state of depression? Zack was cheery after being betrayed, beaten, tested upon, and shot. He did what he had to do and didn't doubt himself at all. I like Cloud, out of the two he's my favorite, but Zack just seems stronger to me.