Something called a "Penning Mix" of 99.75% Ne and .25% Ar is the lowest voltage mix of Ar and Ne, but are there any other gases that can lower this? I know Kr is used in high efficiency T8 fluorescent tubes, but how does that compare to Penning mix?

What is the gas/mixture of gases with the absolute lowest possible voltage drop?

Voltage drop or ignition voltage?

These are different parameters and are not immediately related.

Xenon is usually associated with the lowest voltage drop in the discharge column, but Xe-filled lamps do not necessary have the lowest ignition voltage.

I am mainly talking about voltage drop while running, not ignition voltage.

So Penning mixes are not relevant here. Xenon (and mercury vapor BTW) are your friends