Depends on the vape, and the desired performance.
Are you looking for bigger clouds in fewer draws, or whispy hits with more flavor?
With my E-Nano, I sometimes grind fine with my SCS turned sideways for finer grinding when I desire bigger clouds (to show off for friends who still combust). But when I'm chasing flavor, I like to toss a whole unground nugget in the stem and break it up between hits while stirring.
This yields whispy hits, but the flavor is MUCH better throughout the whole session
(Note, not all vapes can handle whole unground nuggets)
Edit - just saw you said you were looking to maximize your efficiency. I suggest a medium fine grind (not powder, but not too coarse)