Two-Ray Ground Propagation Model or Free Space Propagation Model
If you use the free space model it doesn't account for multipath fading. This isn't a problem in outer space or on point-to-point microwave links using dish antennas but, for more regular earth bound transmissions, you might want to recognize the problem of multipath fading and using a model that takes it into account is going to give a more sensible result.

Having said this, I estimate using the free-space model and add a further loss (about ~20 dB) that is appropriate for taking into account possibility of partial multipath fading. If I think there is the possibility of full cancellation I look for antenna diversity.