Which habitable zones are the best to actually search for life?

Looking to the future, NASA and other space agencies have high hopes for the field of extra-solar planet research. In the past decade, the number of known exoplanets has reached just shy of 4000, and many more are expected to be found once next-generation telescopes are put into service. And with so many exoplanets to study, research goals have slowly shifted away from the process of discovery and toward characterization.