A real gas most closely approaches the behaviour of an ideal gas at

An ideal gas is a hypothetical gas whose pressure, volume and temperature behaviour is completely described by the ideal gas equation. Actually no gas is ideal or perfect in nature. All gases are real gases. Real gases do not obey the ideal gas laws exactly under all conditions of temperature and pressure.

Real gases deviate from ideal behaviour because of mainly two assumptions of "Kinetic theory of gases".
(i)The volume of a gas particle is negligible compared to the volume of the container (while the real gas particle has some significant volume).
(ii)There is no interaction between gaseous particles (while attraction forces exist between real gas particles).

So at lowest pressure and highest temperature, a real gas most closely approaches the behaviour of an ideal gas.