I'd submit that there is no "best." A lot of it is preference and most of the rest depends on other construction, enclosure, crossover, and synergy with the rest of the system. For awhile I thought I hated metal cones, particularly aluminum. Bought a few that seemed striking and wonderful, at first, but then wore my ears down and down. Once that "sound" was set, always seemed to be grating, even in some otherwise really good or fairly expensive speakers on audition. Until I heard the aluminum coned mids in Canton's Reference line and reinforced when I heard the magnesium coned Seas W18. To my ears, those are fantastic.
But, I really like some paper coned mids, and, most may disagree, but I liked some of the plasticky graphite whatever they were called Infinity mids from the 90s. Many others as part of whole speaker, as well.
The cone material is just one factor in the design and finished product and sound, ime.
Planars/ESLs, of course, can sound great, too. Whatever floats one's boat the highest.
But, I really like some paper coned mids, and, most may disagree, but I liked some of the plasticky graphite whatever they were called Infinity mids from the 90s. Many others as part of whole speaker, as well.
The cone material is just one factor in the design and finished product and sound, ime.
Planars/ESLs, of course, can sound great, too. Whatever floats one's boat the highest.