(SPOILER)Early on: Zack, because he's a buff SOLDIER and Cloud's a motion-sick weakling.

Later on: Cloud. Even if you ignore the facts that Cloud has his sword and Zack gets killed, Cloud's Omnislash is enough to take out Sephiroth in the final one-on-one. Sephiroth can kill Zack. Cloud can kill Sephiroth. You do the math.