Naruto would be the sole survivor of this battle. Cloud would die by Sephiroth's hands. He is no true match for Sephiroth. He needed his friends to beat Sephiroth and he defeated Sephiroth a second time because Sephiroth had tooken over Kadaj's weak body and he was not in his own stronger body. Naruto would have killed Sasuke but, he held back to see if Sasuke was truly serious about killing him.
Now we eliminate Cloud and Sasuke. Naruto is far greater then Sephiroth when in his regular form but, in his nine-tailed form then he is insanely stronger then Sephiroth. His speed, power, and overall abilities are overpowered. He could win in his first, second, third, or fourth kyuubi form.
Heres how it would play out. Sasuke would kill Cloud while Sephiroth held off Naruto. After Sasuke killed Cloud, he and Sephiroth would double team Naruto. I'm still not sure if Naruto would take both of them out, but it's very likely. It would have to play out something like this because Cloud just can't compete with Sasuke.
Well, I'd say Naruto would probably be able to kill, if not at least injure, Sasuke rather easily. Cloud against Sephiroth would be a good fight, but in the end, Cloud might be able to win, though I doubt it. Naruto would probably kill Sasuke, and then go kill Seph.
Just to get this clear, Cloud is no match for Sephiroth. Zack couldn't even beat Sephiroth and Zack was way above Cloud in skill and talent. Cloud is no equal to Sephiroth. In KH, they try to have it where Cloud and Sephiroth are equal when they are not. Sephiroth would end Cloud's life the moment the fight started with one swift attack. Cloud is a punk weakling compared to Sephiroth.