Which frequency will have the least attenuation for long-distance skip propagation?
Why Ionospheric absorption, which happens mostly in the D layer, falls off roughly as the inverse square of frequency, so the higher you go the less signal you lose on the way up and back down. The MUF is the highest frequency the ionosphere will still refract back to earth on that path, so operating just under it gives the strongest signal with the least attenuation. Operators often aim for a band near 80 to 90 percent of the MUF for reliable, low-loss skip.
Watch out The choice just above the LUF is the opposite extreme: the Lowest Usable Frequency is set by absorption, so a signal barely above it is being heavily attenuated and barely makes it through. The critical frequency applies to signals sent straight up and is well below the MUF for a long, low-angle path, so those choices would waste the available refraction.
Absorption drops as 1/f^2, so ride high: just below the MUF.