* By sheer number: It's likely nematodes (roundworms). There are estimated to be over 4 quadrillion (4,000,000,000,000,000) of them on Earth!
* By biomass: krill are likely the most common, with an estimated biomass of 500 million tonnes. They're tiny crustaceans found in oceans around the world.
* By species diversity: insects win this one, with over 1 million known species and an estimated 9 million more yet to be discovered.
So, the answer is it depends!