What style of playing is good to study in order to learn more advanced solo guitar techniques?
Fingerstyle & traditional classical is the way to go. These styles are more comprehensive in that they feature the use of all fingers & all the strings being used in a wider variety of combinations. The interplay of bass line, melody line, phrasing, timing & the emphasis on particular parts of a composition are all very important factors. The overall dynamics of expression are given attention. So, there’s a lot a stuff happening in the solo fingerstyle area & there is some very serious discipline required to learn it … but the results are certainly well worth it.