If xenon didn’t exist in nature then DARPA would fund someone to develop it – because it is that good for propulsion.  When stored under moderate pressure Xe is a supercritical fluid – meaning it has the high density associated with a liquid, yet it fills the container with no free surface and no sloshing.  Krypton and argon have been used in place of Xe for some missions with the goal of reducing cost.  The replacement is not without trade-offs and the propellant cost savings can be erased by system performance impacts that have associated costs of their own.  Kr and Ar store at very low density, thus much larger storage volumes are required for the same propellant mass.  The tanks must be at higher pressure and so require thicker walls which, when taken together with the larger size of the tank, translates to increased dry mass.  Kr and Ar have lower propulsive performance than Xe and they cause faster erosion of thruster components which reduces the lifetime and achievable total impulse available to maneuver the spacecraft.  Orbion offers the Aurora system for operation on either Xe or Kr.  The slider and graphic show volumetric trades (at 20 deg. C) to store 15 kg of propellant.  Note that flight systems typically require tanks capable of 1.5X maximum expected operating pressure and that storage pressure increases with tank temperature.