In a straight fight, no tricks or preparation, Squall would win. Aura + Lionheart. He can use his Limit Break as many damned times as he wants. Cloud can only use his Omnislash when his Limit Meter is filled. That's a serious disadvantage.

If there was prep time allowed, It would be a close call between Zidane and Squall. Both are cunning and both could figure out very effective ways of outmaneuvering their opponents so they were the only two left.

Tidus would go out first because he'd spend the first five minutes of the fight complaining.

Cloud would go out second because he's a weak-willed sissy.

In the end, it would come down to Squall and Zidane, but I don't honestly think Zidane could win. It would take some serious planning to outmatch Squall.