The IAU's list of requirements to be a planet is:

• Orbit a star
• Be big enough that it becomes round
• Clear the neighbourhood (meaning you're way bigger than anything else in your orbit)

The last one is the one that disqualifies Pluto. For comparison, Pluto is roughly 8% of the mass of the other stuff in its orbit (not including Neptune, given that their orbits cross), whereas Neptune is thousands of times more than the rest of its orbit. The closest non-planet to meeting this criterion is Ceres, which is roughly one third of the rest of its orbit (in the asteroid belt).

Based on this list, I think Jupiter's four biggest moons (Io, Europa, Ganymede, and Callisto) would make the cut.