Cloud because he was able to defeat Sephiroth, while Zack was pretty much owned by the same Sephiroth.

We'll seeing as how Zack was the one who carried clouds sorry ass out of the shinra mansion in the first place and zack was the one who actully made it into solider and he stuck with cloud to the end, id say Zack is the stronger of the two

Umm...actually, yeah, that's exactly what Sephiroth does. Not once does he take Cloud seriously while fighting him one-on-one. He leaves himself wide open to attack, fails to make use of his entire range of abilities, refuses to take an opportunity to just end the fight quickly, and simply doesn't consider Cloud a threat. This is most apparent during their Advent Children fight, in which Sephiroth actually has Cloud pinned to the ground, barely able to lift his sword. So what does he do? He stabs him through the shoulder. Had he impaled one of Cloud's vital organs, the fight would've been over, but Sephiroth chose to go for the shoulder to prolong Cloud's suffering.

Even without taking that into consideration, Sephiroth could've simply used telekinesis to hold Cloud in place from the get-go. Last time, it took the entire party to break free of Sephiroth's telekinesis. Even if Cloud's strength doubled since the end of FFVII, he still wouldn't be able to free himself from Sephiroth. However, that's not all Sephiroth could've done. He could have simply attacked Cloud from afar with Iakiri, speedblitzed him with Octoslash, obliterated him with magic, turned intangible to evade Cloud's attacks, the list goes on. Yet, despite having all of these options available to him, Sephiroth chose to attack with basic swordsmanship. He wanted to prove to Cloud that, even on his own terms, Sephiroth was still far superior.

His fight with Sephiroth in Nibelheim is no different. Of course, it's too short for Sephiroth to have toyed with Cloud, but the fact remains that, ahd it been a fair fight, Sephiroth would've curbstomped him. By the time Sephiroth even realizes that Cloud is at the Reactor, he'd already been stabbed through the back with the Buster Sword. Now, for all intents and purposes, his spine should've been severed, but Nomura's logic says that it wasn't, so we'll just have to go with that. Either way, Sephiroth was obviously seriously injured considering Cloud had just cheapshotted him from behind with a weapon that's probably bigger than he is.

So Cloud tries to finish him off on the catwalk, but he is promptly impaled and lifted into the air. In a fit of nervous adrenaline, Cloud is able to grab a hold of the Masamune and pull himself further into it, allowing him to use the sword as a lever. Sephiroth, stunned and probably dying of bloodloss, is quickly thrown into the Lifestream (according to Crisis Core. I realize that Last Order says otherwise, but I'm going by the most recent source). This is why Sephiroth hates Cloud so much and wants nothing more than to see him suffer.

Now, their duel in the Lifestream is harder to judge for several reasons. The first and most important reason is that it doesn't take place in the physical realm. It's strictly metaphysical, so Cloud and Sephiroth's respective physical abilities may not remain consistent in the spirit realm. Perhaps Cloud has an easier time gathering the Spirit Energy neccessary to use Omnislash in the spirit realm (which would make perfect sense, actually...). The second reason is that Sephiroth had just gotten the ever-loving crap beaten out of him by AVALANCHE. His body had been ripped to shreds in the physical realm, which would probably have a pretty drastic effect on his stamina in the spiritual realm.

So...err...yeah, that's pretty much it. Sephiroth does hand Cloud the win every time they fight.

That doesn't change the fact that Cloud was able to do what Zack could not...and that's kill Sephiroth.

Yes, Cloud kills Sephiroth, but it's not because he's at all superior to him. If Sephiroth were to go all-out against Cloud, I can guarentee that the fight would be over with in a matter of seconds. Fighting against a Sephiroth who actually takes him seriously, Cloud would do no better than Zack did. He'd either be overwhelmed, speedblitzed, or ripped apart by telekinesis before he knew what hit him. There's just no other explanation. The creators even say that Sephiroth wasn't exerting any effort AND that he's the strongest entity in the Final Fantasy VII Universe. Cloud's victories against Sephiroth mean absolutely nothing because Sephiroth is never using any semblance of his full strength.

If I came up behind the world's greatest swordsman while he was talking to his friend and stabbed him through the spine with a katana, would I be considered the greatest swordsman in the world? The simple (and only) answer is "no." To be considered a better fighter than someone in the conventional sense, you must best them in a fair fight. Cloud has never done that to Sephiroth, and neither has Zack. Since they each did equally pathetic against Sephy, their respective fights can't be used as evidence to claim that either one is stronger.

Very well but still if you don't consider the fight against Sephiroth to show that Cloud is the strongest, then don't forget about all of Clouds acomplishements he is able to fight both Loz, Kadaj and Yazoo alone. He beats Kadaj one on one, and i doubt Kadaj was holding back...

He destroys Bahamut (granted he had a little help from his friends).

Zack, kills Shinra soldiers and that's it.