Donald because he can cast vast amounts of magic and is also very protective about his allies. He can cast Cure in a short amount of time which is a big plus. Sora is the typical hero, which I'm not going to join the bandwagon. Goofy is just useless in KH1. He doesn't have any good special attacks and his standard attacks are weak.
It'll be kinda unfair to say Sora since he can pretty much do everything. Goofy is pretty strong but he dies quite a lot on me. Donald is always a nice help since he can heal you when you're in danger and has some good range so he doesn't die quite as often as Goofy who pretty much as to charge in to attack.
Donald is helpful with his healing but he wastes his magic a lot. Goofy does some good damage and high HP and Sora is just balanced. I guess Goofy then.
Goofy. Donald always died on me first, and I'm not one of those people that adore Sora, exactly. Goofy always had a sort of charm to him, and I'm sure if he was playable, his high defense and great special moves would destroy EVERYTHING.