Is 'most' in this sentence a determiner or pronoun or adverb?

if you drop / of/ you would need to say "that most valued commodity"

because in "that most valued Of commodities", of = among several

/most/ is used to form the superlative of adjectives which don;'t take -er, -est in the comparative and superlative - such as weak, weaker, weakest; valued has to be valued, more valued, most valued. In this context more and most are adverbs.