Several organisms are plentiful on earth. When speaking of "creatures" the group known as insects is the largest group of tangible creatures.
Plankton
Unicellular- organisms include bacteria, the most numerous organisms on Earth. Multicellular- organisms are composed of many sells and they lie
diatoms
Cyanobacteria also called as blue green algae. They are the photosynthetic prokaryotes. They are found in both marine and freshwater. They are probably the most numerous taxon to have ever existed on earth and the first organism know to have produced oxygen.
Bacteria & Insects.
Bacteria, the most numerous organisms on Earth , are unicellular organisms - just had this question on a homework
Fish
I assume you meant single "celled" organism. It is an organism (life form) that consists of only a sigle cell as opposed to most all other lifeforms on earth which are comprised of many (billions) of cells.
Pizza
fish
The answer is the easiest answer..........BACTERIA
single-cell
Phytoplankton.