.30-40 Krag. However, .30-40 Krag brass is likely to be harder to find and more expensive than .303 British.
Years ago, I was given some .30-40 Krag brass. I fire formed them to .303 in a Pattern 14 Enfield, using the "unique, cream of wheat, and drop of elmer's glue in the neck", method. The fire forming worked fine but left a big mess in the rifle.