Vesta is the asteroid belt's second largest object, but figuring out what it is has driven astronomers crazy. Is it an asteroid? A planet, perhaps a protoplanet? Whatever the answer, Vesta might just be the solar system's most important object.
Our current planet classification system isn't perfect, and it's still more than a little controversial with astronomers and laypeople alike - just look at the recent (flawed) assertion that Earth might need to be demoted for ample evidence of that - and yet it's not terrible at dividing the solar system's many bodies into some broadly useful categories.