I'm going with Riku on this one.
Riku's got the Darkness powers. I mean Dark Aura, Dark Firaga, Malestorm, not to mention his 3 hit slash which knocks you unconsious for a couple seconds and Dark Break
Leon can extend his sword's size and make it stronger as well as fire a fireball from his blade. He is more experienced true. But Riku's speed far surpasses Leon's and I doubt Leon could block a barrage such as Dark Aura easy.
That and Dark Firaga basically cancels out Leon's fire ball attack.
That's just Riku as a child, as an adult he'd beat Leon hands down. Now Cloud's another story