The question is very ambiguous as there are a number of different types of planets, there are planets in the solar system, gas planets, exoplanets etc. So below is the largest in each group of planets.
The largest planet of any type (rock and gas) in the solar system is Jupiter, it is over 11 (eleven) times the diameter of the Earth. Jupiter is a gas giant planet, with a possible rocky core.
Earth is the largest rocky planet in the solar system, it is the planet that we live on. It is the only planet in the solar system, galaxy and to make it even more outstanding, the universe that humans have stepped foot on. the moon is not a planet as it orbits an object that orbits a star.
The largest Exoplanet, that is a planet that is orbiting a star other than our own Sun is Wasp 12b, exoplanets are being discovered all the time and this planet won't hold the title for too long. Wasp 12b is in the constellation of Auriga, the Charioteer
The largest exoplanet in comparison to its star is NGTS-1 orbiting a star in Columba the Dove.