Most common protozoan parasite is
Toxoplasma gondii is an obligate intracellular parasitic one-celled eukaryote  that causes the infectious disease toxoplasmosis  domestic cats, are the only known definitive hosts in which the parasite may undergo sexual reproduction.
In humans, T. gondii is one of the most common parasites in developed countries.
Toxoplasma gondii
Trypanosoma
Plasmodium
All of the above