Well, Oswald, of course, although he did get a big boost in the Epic Mickey games.
Second choice would be Horace Horsecollar. A lot of people don't realize what an important character he was during Disney's beginnings. In the classic Mickey Mouse comic strips and books, HE was Mickey's sidekick, not Goofy. He and Clarabelle Cow were the older, more mature couple in comparison to Mickey and Minnie, and they all palled around together and went on adventures. They were a good fit, and warm and funny as well. Since Horace was more adult and practical, Mickey got to be more boyish and mischievous, parts of his personality that nearly everyone says are missing today. Later, when Goofy replaced Horace, then suddenly Mickey had to be the responsible one, and couldn't afford to be mischievous and playful, and so his character got diluted into blandness. I really wish Disney would bring back Horace and Clarabelle - and draw them in their classic style:
And let Mickey be playful and funny again.