Let's set aside the obvious corrosion resitance factor for a moment...

If building a match/varmint rifle, which type of steel would you choose for the action and WHY?

(ie. longevity, galling, ease of machining, etc.)

These days a majority of the custom benchrest actions are SS. That's not saying that the material is better but the actions guys like Farley, Nesika, Kelbly, BAT, etc. are superb foundations for super accurate competition rifles.

There is really only one good gunsmith in my country, and he says cro-mo is best for receivers and stainless for barrels.

Although I am certain the stainless US custom actions are as good as anything out there.