Hi Georgia and Cora!! Great question! This ones easy! LIFE!!!! Earth is super special because it has life on it. In no other place in the universe do we know for sure that that has happened.
But, if we are talking about the planet itself – I would say the Moon, and plate tectonics. Both of those things are pretty unique in our solar system. Drew mentioned that it is weird we have a moon at all. It really is! There are some other IAS questions in our zone about the moon – check them out. Also, the Earth is the only planetary body we know has tectonics. Tectonics may be crucial for life, all that recycling of the Earth’s crust also cycles essential elements through the mantle and up to the surface that are essential for biology. Many believe life would not exist here without tectonics!!