In short equator is defined as the line passing from the center of earth which divides the entire planet into two parts , northern hemisphere and southern hemisphere. for more information you can refer to wikipedia
so the question is which countries are most nearest to the equator.
There are total 14 countries nearest to the line of equator or some of them are passing through the equator line.
south of Baker Island
São Tomé and Príncipe Ilhéu das Rolas
Atlantic Ocean Gulf of Guinea
Republic of the Congo
Democratic Republic of the Congo
Gulf of Tomini
south of Aranuka
hope you will find this information interesting , this is updated and knowledgeable fact.