The word "BEST" can be two things: It can be the either the superlative of the adjective " GOOD" or the superlative of the adverb "WELL". In that case, it's clearly an adverd, since it modifies the passive verb "IS MEASURED". While adjectives modify the meaning of nouns, adverbs modify the meaning of verbs (and of other categories, too). However, adjectives don't usually modify the meaning of verbs, so this is not "BEST" adjective, it's "BEST" adverb, in this case, and it means: "WELL AT THE MAXIMUM GRADE", so it means "IS MEASURED WELL AT THE MAXIMUM GRADE", well at the maximum possible, imaginable grade. As to "WHICH", it's difficult to say which word is referred to, I didn't catch the exact meaning of the passage, maybe you should add something. However, "WHICH" comes after a COMMA in that sentence. When that happens, "WHICH" usually (but not always) refers to the WHOLE SENTENCE that comes before it. I mean, the whole sentence is the "antecedent" of "which". In a case like this it would mean: "...and this thing (the idea expressed by the whole previous sentence) is best measured...etc.

Thank you for your comment.

which is the scale of our being