Eris is more massive than Pluto. After being reclassified from planet to “Kuiper Belt Object”, Astronomers have now determined that Pluto is not even the largest such object. Eris, being heavier and further away from the Sun, has the bragging rights, at least for now.

Looking at it the other way, if you insist that Pluto really is a planet, you now have to admit that Eris is as well.