Literature is the reflection of human activity by the finest minds.
Literal
Literature [topic-は], most excellent spirit [possessive-の] holders [agent-による] human-activity [possessive-の] reflection is.
~による means 'by' (agent in a formal construction). 持ち主 means 'possessor/holder.' The whole phrase もっともすぐれた精神の持ち主 is a relative clause meaning 'those who possess the finest minds.' A dense, literary definition.