We present here an optimal list of targets observable by ARIEL (Atmospheric Remote-sensing Exoplanet Large survey), one of the three European Space Agency M4 mission candidates competing for a launch in 2026. In aprticular, we describe the assumptions made to estimate an optimal sample of exoplanets - including both the already known exoplanets and the "expected" ones yet to be discovered - observable by ARIEL and define a realistic mission scenario. The current ARIEL design enables the observation of 1000 planets during its four year mission lifetime. This nominal list of planets is expected to evolve over the years depending on the new exoplanet discoveries. Among the 1000 planets a good fraction of super-Earths and earth-sized planets will be included. While ARIEL will focus mainly on warm and hot planets, we briefly discuss its capability to study temperate planets around cool dwarfs like the ones orbiting TRAPPIST1.