For long-term settlement... Obviously breathable air would be great. Decent air pressure and temperature would make growing food easier, but nothing hits all those markers in our solar system. A magnetosphere to block radiation... I think we're just so perfectly adapted for the very specific conditions of our planet that it would be really difficult to find another planet that we're equally adapted to.
RE: The Top 5 Places We Could Colonize In Our Solar System | Answers With Joe