Europa and Enceladus
Europa and Enceladus are both huge candidates for extra terrestrial life. Europa is the smallest Galilean moon but still is one of the biggest moons in the solar system. Enceladus is the sixth biggest moon of Saturn and is considered the source of the E ring. Europa's surface is full of ice and silicate rocks and it probably have an iron core. Its surface is smooth and fairly young for a moon. There is a chance that there might be an ocean underneath its surface.Jupiter's huge gravitational field would cause enough friction to melt ice into an ocean.
Europa is tidally buckled with Jupiter. One side is always facing jupiter while the other side is always facing the opposite side. Europa water (if there is any) would be deep under its surface . Only information of Europa is by fly-by missions.
Enceladus reflects most of the light it receives and is the brightest object in solar system after the sun. Enceladus has Cryovolcanoes near the south pole shooting out plumes of hot liquid full of organic materials. Some freeze and fall as ice while some part feed the E ring while the rest goes to Saturn.
The Cassini spacecraft took some samples and the equipment onboard wasn't strong enough to identify all the organic materials. The surface temperature is -200 degrees celsius. It was discovered by William Herschel. There is the possibility of thermophiles or hyperthermophiles living in Enceladus. These are microbes whose name literally means heat lovers.
All information of Enceladus is from fly-by missions.