SF6
The "best" inflation gas for aircraft tires from a performance point of view is sulfuric hexafluoride, SF6, a gas which is normally used in gas-insulated switchgear for high voltage applications (>100 kV) world wide. SF6 has excellent dielectric properties and its extinguishing capabilities are outstanding and outperforming every "competitor". Combine this with a really big molecule, Nitrogen can dust off....
Well, on the downside, SF6 is a "climate killer", so no good and therefore no large scale use in aircraft tires.
I stand to be corrected that the space shuttle still uses this gas in its tires ?