An important and fundamental fact from algebra and the study of groups is that every coset has the same number of elements (which leads directly to the statement of Lagrange's Theorem.
Lagrange's Theorem says that if $H$ is a subgroup of $G$, then the number of cosets is $\dfrac{|G|}{|H|}$, and each coset is associated to $|H|$ different elements of $G$.
So if you want to have cosets that have only one element, then you are quotienting by only the trivial subgroup. If you demand that all subgroups yield quotients that have cosets with only one element, then you must be starting with the trivial group.