Under what conditions is a gas closest to properties of an ideal gas?
At high temperature and low pressure.
Ideal gas is that which obeys Gas Laws at all temperature and pressure but real gases do not obey gas laws at all temperature and pressure.
Real gases only obey Gas Laws at high temperature and low pressure. Because at these conditions the gas molecules have much empty spaces between them and the actual volume can be negligible. And also the intermolecular forces at these conditions can not be felt by molecules because they are so far apart from each other. That's why intermolecular forces are also negligible.