Of course, what's "best" depends on your goals.
For best efficiency, a mu ratio of 0.35 turned out in the tests of the 1930's to be best. The "mu ratio" is the ratio of the aircraft's forward speed to the blades' tip speed. You'll need to convert back and forth between miles per hour and feet per second, and also know how to calculate tip speed from RPM and diameter.
Obviously, a rotor without collective pitch can only be optimized for one airspeed using this rule.
For various reasons, people may depart from the rule.
Bensen's gyros ran at about mu=0.2. That is, their rotor RPM was quite high for the aircraft's low cruising airspeed. Perhaps this is just the way things happened to work out. Or perhaps Bensen, like Vance, wanted some extra RRPM margin above the low RPM at which catastrophic flapping would occur. Bensen blades lost RRPM FAST when unloaded, because they were light and draggy. We all noticed increases in performance when we slowed down our Bensen rotors a bit by adding more pitch, diameter, or both. Non-Bensen (and post-Bensen) blades tended to have longer blade chords, which also decreases RPM and moves in the right direction from the mu-ratio viewpoint.
High RPM blades have lower amounts of rotor damping than slow ones of the same mass and dimensions. Rotor damping is the tendency of the rotor to lag behind cyclic control inputs. High-RPM blades respond faster, with less stick back-pressure, to a control input. People tend to find that Bensen blades have "light, snappy' response compared, say, to the slower, heavier McCutchens.
Selecting a RRPM for a chosen cruise speed is a compromise between a high enough tip speed to minimize the draggy, stalled portion of the retreating blade and low enough tip speed to avoid excess drag on the advancing blade tip (where the blades' airspeed is highest). .
