i think OP team would win with high difficulty,they have higher stamina and IMO better swordmanship
also,cloud beating sephirot is pure PIS
zack died battling weaklings :/
Mihawk's not got many speed showings at all but as I recall he could easily react to and deflect bullets. As well as this he was able to keep Gear 2 Luffy on the defensive and he took out Daz Bones in 1 second. He'd also have haki which would give him pre-cog like abilities, which would make hitting him nearly impossible for people who don't have haki.
As for Cloud, what are his speed showings? I don't think Cloud would be anything more than super sonic, if even that. He always uses Fenrir to get around when he needs to get somewhere fast. However I imagine his real strength would come from his use of Materia. He's been shown to use powerful materia like firaga and thundaga which are probably building level. He can can also likely slice through buildings with ease, similar to how Sephiroth could. After all he did slice right through Bahamut in AC. He's also got a number of powerful limit breaks.
But like I said, I really don't recall Cloud ever showing anything that can match up to Mihawk's showings. Heck Mihawk even crosses the oceans on what is effectively just a piece of wood and took out an entire fleet of ships in minutes, just because he was bored.
I'd say Cloud vs Pre-Timeskip Zoro at around about the Thriller Bark arc or something would be a close battle but after that OP just became far too powerful.
I'm not too familiar with Zack, but I assume he's not as powerful as Cloud was by AC.
@SpeedForceSpider: Is he? As far as I can remember he's never shown anything so impressive during any of his fights. Also Cloud in AC has been said to be his most powerful incarnation and he didn't look like he'd be able to compete against someone of Mihawk's calibur in the film.