Donald
Donald is the mage of the group, making up for Goofy's lack of spellcasting abilities by flinging around enough pyrotechnics for the both of them. When he runs out of MP, which he inevitably will, his staff comes in useful as a melee tool and he is surprisingly good at wielding it. However, magic is his forte and so his MP should be maintained with Ethers; just be sure not to set them to auto-reload as these things run out very quickly.
Now for the most important part: Donald knows Cure. He is ready to use it at any time and at any level, so keeping him close by will save Sora's life on numerous occasions. Cure should remain set to free use unless you have a very good reason to change it. Drop other magics down to occasional or rare to ensure he always has enough MP to cast it. And always, always monitor his HP and listen out for his quacks to signal that he has been KO'd so that you know to fight defensively until he recovers.
