best rpm for most energy efficiency
There are a few studies which show that lower pedaling rate is more efficient (i.e. consumes less oxygen/fuel for the same power output):
1, 2, 3
However, it stands to reason that it’s more tiring for your muscles because they have to produce more force/torque to achieve the same power output.
I think your body is pretty good at picking the right cadence for the current circumstances. Most cyclists automatically pedal at around 90rpm when riding at moderate or high intensity. That being said, I think it can still make sense to train at very low or high cadence from time to time. It’s also a good idea to have the right gear range and gear steps for the terrain you are riding.