Helium is ideal. Nitrogen is heavier than helium and argon is heavier than both.
As stated above, many people have failed with argon.
They all have the same capacity to cause death by hypoxia, but argon is so heavy it tends to drop below air, making the exit bag method ineffective even with a higher than 15L min flow rate.
I haven't seen much on trying it with a mask though.
Helium > Nitrogen > Argon