Obviously Europa and Titan are prime candidates for the discovery of alien life forms in this solar system. Humans should make every effort to explore these moons in our quest for understanding the universe.
From an engineering standpoint, Titan is the best choice because it has an atmosphere that the Huygens probe already successfully used for aerobraking. Enceladus and Europa have no atmosphere so you have to carry fuel to brake and land. Then you're faced with many kilometres of ice to tunnel through before getting to the interesting bits. Biologically, the scientists so far consider the ice moons as being more likely to harbour life. If the money were avaiable, we should visit all 3 -- and quickly because I'm too old to wait 20 years.