Squall since the junction system is way overpowered. Auto-Haste and 255 speed makes him faster than most, if not, all other characters. Lion Heart would easily take care of Wakka's 99,999 HP and due to his massive speed (510 with Auto-Haste), Squall would always attack first. Not to mention you could equip him with Initiative to have the ATB bar maxed at the start.
Or you could do the same with Selphie and use The End instead of Lion Heart. 1-hit KO on anyone unless they're undead. Actually nah, that'd be crap. Just use Zombie on yourself, easy.
Lol at all the Cloud fanboys

. Junction system > materia system. The FF9 chars have no chance vs Squall either.
Judging by what Tom is saying, the FFX chars are the only ones which would give Squall a run for his money, but I'd still say Squall has the advantage. I'm not really THAT experienced with FFX though, I played it to the end and got bored after fighting Sin for the last time. I never got my characters powered up enough to see their true potential and didn't really do many sidequests.