What is the optimum solar proxy for long-term ionospheric studies?

For long-term studies as ionospheric trends investigations we have to use proxies of solar activity, because homogenous and sufficiently long data series of solar ionizing radiation are not available. Here I deal with selection of the optimum solar proxy for yearly average and monthly median values near noon (11-13 LT). Six solar proxies are used, F10.7, F30, Mg II, He II, Fα (solar H Lyman alpha flux) and R (sunspot number), foF2 from European ionosondes Juliusruh, Pruhonice and Rome, and foE from Chilton and Juliusruh over the period 1976-2019. For yearly values Mg II is the optimum proxy (but it is available only since late 1978) for foF2, with F30 being the second best. For foE the optimum proxy appears to be F10.7. For monthly medians of January, April, July and October the general pattern is the same as for yearly values.