Which is the closest galaxy?
The Andromeda galaxy (M31) is the closest large galaxy to us.
Short answer: The Andromeda galaxy is the closest major galaxy to the Milky Way but there are over 80 small dwarf galaxies that are closer.
Long answer: Like many questions in astronomy, this question is not as simple as it seems. While the Andromeda galaxy is usually said to the the closest galaxy to ours (the Milky Way), this isn't technically true. There are many smaller dwarf galaxies that are closer. To complicate the situation further, astronomers disagree about some of these galaxies which may not even be galaxies at all.
The Milky Way is the galaxy that we are part of, so if you're asking which is the closest galaxy to Earth, you could say that it's the Milky Way.
Outside than the Milky Way, the Canis Major Dwarf galaxy could be the closest galaxy to Earth. However, astronomers aren't sure if it's really a galaxy or if it's just part of the Milky Way.
Other nearby dwarf galaxies include Draco II, Tucana III, Segue 1 and the Sagittarius Dwarf Spheroidal Galaxy.
The Andromeda galaxy is definitely the closest spiral galaxy to the Milky Way at 2.5 million light-years distant.