While there is no 'perfect' operating frequency, I have to admit I have a liking for the 6.59 kHz models White's has offered in more recent years.
While there is some merit to the benefits of using higher or lower frequencies for coin hunting, a fair amount of what is discussed is based on 'theory.' There is a lot more involved in the process of metal target location and discrimination than simply the operating frequency.
Come March I will wrap up my first forty-two years in this great hobby and as I reflect back on most of my better finds made since the mid-to-late '70s when we started using VLF (Ground Balanced), VLF/TR-Disc., and later the VLF-Disc. (motion) detectors, the bulk of my better finds have been made using models that operates in the 6.59 kHz through 15 kHz range.
During the 'reflecting' the one most important key to success has not been the operating frequency, but the site selection. LOCATION has played the biggest role in my success, followed by operator (knowledge, skill, patience, detector set-up) and a close third factor is the detector (operating frequency, discrimination characteristics, sweep speed requirement/allowance, search coil selection, ground adjust ability, etc.).
Considering the models of detectors that provided me with the highest volume of jewelry and coins (both new and old), from a variety of sites (also both new and old), in relation to the time used, I've had a good mix of frequencies used. The 6.59 kHz White's XLT, XL Pro, Classic III SL, and Classic ID/IDX Pro; 15 kHz Garrett Ground Hog, Gold Mountain VIP, King Cobra and GMT-1650; 10 kHz to 12.5 kHz Tesoro Silver Sabre, Inca, Silver Sabre II, Silver Sabre