If we're talking about a single species, the most populous organism on earth is likely the _bacteria_. They are found in virtually every environment on Earth, and there are estimated to be 5 nonillion** (5 followed by 30 zeros) bacteria on Earth.
However, if we consider individual organisms, it's likely a _phytoplankton_, a type of algae that lives in the ocean. There are estimated to be 10^28** phytoplankton in the ocean at any given time.
So, the answer depends on how you define "organism" and "populous." Either way, it's safe to say that microscopic life dominates the Earth in terms of sheer numbers.
