Squall wins, hands down. The Lion Heart beats both the Ultimate Weapon and the Caladbolg. Tidus is agile, but Gunblades always hit in FF8 - and even hit hard because of the gun part of the blade. I would NOT want to get struck by that.

Comparing the Limit Breaks of the three, we're in a bit of a ditch. The competition is inheritly badly balanced because X has Break Damage Limit and neither Cloud nor Squall do. Because of the different postgame standards, I'm gonna say hitting for 99999 in X counts as hitting for 9999 in VII and VIII. By this, Squall wins effortlessly again, because not only does Lion Heart hit more times than both Omnislash and Blitz Ace, its hits actually deal a MULTIPLE of the base attack damage rather than a fraction like Omnislash and Blitz Ace.
Besides, FF VIII's Limit Break system allows for effortless spamming unlike either of the other games. Squall essentially gets multiple free hits every turn when compared to the other two - and that without assuming he has an uberpowerful build.

Overall, I have to say Squall would win, though both of the others would certainly put up a fight.