It is whatever becomes comfortable for the particular conditions you happen to be riding on.
If you are riding in a bunch and need to follow another wheel, then a lower cadence will help you accelerate and is easier to respond to changing speeds.
If time trial, you don't have to accelerate, just keeping a steady speed, then lower cadences are effective.
Used to have a theory - if your legs are hurting, change down and if your lungs are hurting, change up.
I do a lot of fixed/ss, so in a ride, I end up riding at both ends of the cadence scale. This helps to get to know what is effective, but it is not without pain!
Cheers Keith