* Bacteria: Bacteria are prokaryotes, meaning they lack a nucleus and other membrane-bound organelles. They reproduce asexually through binary fission, where the cell divides into two identical daughter cells.
* Amoeba: Amoeba are protozoa that are known for their changing shape. They reproduce asexually through binary fission.
* Paramecium: Paramecium are another type of protozoa. They reproduce asexually through binary fission, but can also reproduce sexually through conjugation.
So, while there isn't a single "most common" organism, these are some of the well-known examples of single-celled organisms that use fission for reproduction.
