303 turns very nicely.
304 is harder
17-4 is hard, but I really like it.
That's the short answer from my limited experience.
Are there not charts of 'machinability' that include stainless flavors?
Compare all the standard features we've discussed here of late for comparability regular steels when it comes to design. Stainless steels are so for one reason or another - usually corrosion or abrasion resistance, not for strength or ductility. So check all the features.