Which animal is the MOST common in the animal kingdom?

Arthropoda make up about 2/3 of the earths animal population.

Examples of Arthropoda are: Grasshoppers,Bees,Butterflys,Centipede, Lobsters, Crabs etc

They are defined for their Exoskeletons, single jointed legs (kinda like our elbows), segmented body called metameres that are HIGHLY adapted to suit the organism.