Question

What is an ideal gas, and what is the real gas that behaves most ideally?

Answer 1

As ideal gas is a theoretical entity that behaves "ideally". Helium gas is a real gas that should best fit this ideal.

Explanation:

Ideal gases are conceived to have little or no interaction between molecules. The volume of the gas is considered to be negligible. Ideal gas molecules are regarded as small indivisible spheres, whose collisions are perfectly elastic.

Real gases approximate ideal gas behaviour at moderate temperatures and pressures. And helium, (at moderate temperatures and pressures) should have behaviour that closely approximates the ideal. Of course at high pressures and low temperatures, helium will fail to behave ideally.