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.
And how do you figure that? He was the most respected member of Tantalus besides Baku, and they were one of the most notorious gangs of thieves in the world. You can't be a master thief and not be a cunning strategist. Squall is a military strategist, Zidane is a mercenary strategist. Squall's advantages lie in outmaneuvering an army, or team of opponents. Zidane's advantages lie in outmaneuvering a single opponent or a small group. Zidane has the obvious advantage in that situation.
1) Technically, the same could be said for Squall. They never really showed him using any extreme tactical knowledge, it was just implied in his character.
2) Squall never showed the ability to use the Gunblade as a ranged weapon. The actual gun portion of the weapon is used to amplify the damage dealt with a melee attack. There's no way to even know if it would fire straight if he tried aiming it at someone and picking them off at a distance.
Well, Squall has discipline. He's been raised in a military institute.
Zidane can get serious, but it's not really in his nature.
Zidane has the advantage of havinga two-bladed weaon. He could block Squall with one ege and quickly rotate it and slash Squall with the other side.
Squall still has his magic powers. Zidane doesn't. Squall could just cast Ultima, Holy and Flare to beat the living hell out of Zidane.
And, I am just assuming, but I believe that Squall is the stronger of the two. No real proof, just something I think makes sense. Zidane looks to be more agile and quick, not a powerhouse.
And again, as I said, it would be an uphill battle the entire way in order for Zidane to beat Squall. Squall has the overwhelming advantage of power, combined with the fact that the Junction system allows Squall to become nearly unbeatable with the right magical combinations. But, bar none, out of the other three, Zidane would be the toughest opponent Squall would have to take down.