Let's see... (goes with the main characters he actually remembers *doesn't really remember Cecil well*)

I choose Squall and no, I'm not going on in-game battle stats or whatever (hp, damage limit or whatever).

Squall seems to have the most battle experiance out of most of the main characters (I'm going with FFVII-FFX, since FFVI didn't really have a main character yet a main focus).

It's goes like this for me: Squall, Cloud, Zidane, Tidus.

He's a bit more fit for combat than Cloud, seeing as how Squall was, in a way, more professionally trained than Cloud was <spoiler> (seeing as how Cloud was only a guard and didn't receive enough experiance to match a soldier)</spoiler>

Cloud, like Zidane and Tidus, has a more of a self-taught style and very little on-field combat experiance (well, his style is more like mimicing what he saw in battle and improving over time).

Zidane was trained by a band of thieves (most likely anyway, I know it doesn't say he was in the game), and if he wasn't he learned from being put in situations that made him rely on his swordsmanship rather than his speed. This, in my view, can't compare to the training Squall and Cloud had received.

Tidus is a different story. He was handed a sword and shoved into battle by Auron, never actually trained to use a sword by anyone. Compared to everyone else, even at the end of the game, his swordsmanship is a bit lacking and some could even say somewhat sloppy. Some would even say that he relies more on his speed than his swing.

If you want to talk magic:
Cloud and Tidus have the advantage. Cloud is the one capable of having Time, Ice, Fire, Earth, Lightning, etc. and Summoning which basically over powers Tidus' Time Magic, so Cloud is first in such a category, then Tidus (his Haste makes him quite a threat when combining it with his speed).

Sure Squall was most likely trained to handle magic and summoning, but the fact that he has to draw to gain magic enables him to be open in battle. Plus it's also possible that the draw might fail. Zidane basically doesn't have any magic.

If you want to move on in battle stats (limits, stats, battle system, damage)
Tidus would win this round, his break damage limit, hitting for 99999 and his overdrive, Blitz Ace (add on the break damage limit), would enable him to over-power Cloud, Squall and Zidane. In fact, Energy Rain and Slice and Dice is strong, if connected with break damage limit, can easily wipe out those three.

Now, when Squall and Cloud come along, it's odd. Squall deals more damage than Cloud, but Cloud hits multiple enemies (if I remember correctly), the same with Zidane (if I remember correctly). So Squall would only be able to concentrate on one enemy, while Cloud could connect with everyone, hitting for Max Damage.

*shrugs* This is all what I think though.