The choice of dielectric is largely determined by the temperature stability required:

NPO (COG) Ultra stable Classa??dielectric, with negligible dependence of electrical properties on temperature, voltage, frequency and time, Used in circuits requiring stable performance.
X7R Stable Class a?!dielectric, with predictable change of properties with temperature, voltage, frequency and time. Used 5 Uas blocking, coupling, by-passing and frequency discriminating elements. This dielectric is ferroelectric and offers higher capacitance ranges than Classa??.
Y5V(Z5U) General purpose Class a?!dielectric with highest dielectric constant and greater variation of properties with temperature and test conditions. Very high capacitance per unit volume and suited for application as well as filtering, transient suppression blocking, and charge storage application.