IMHO Visionary doesn't really match Vordenker ("Wer Visionen hat, sollte zum Arzt gehen" Helmut Schmidt), Mastermind is much weaker than Vordenker. Maybe I explained Vordenker not well enough, but this is hard as the meaning in English seems not to exist according to Wikipedia results. It's often used in a biographical and historical context and user phenry seems spot on here, it simply doesn't exist in English.
I'm asking here, because I want a feedback, how above terms in the title are understood actually by most Germans. Are they basically synonyms? What's next to Vordenker? What are the nuances between my current favorites Pionier and Vorreiter esp. when judging the historical influence and importance of a single person in retrospect?
You can give a own english translation of Vordenker as a side note or better just vote the available on EL&U, but that's not the question here. Answers in English please and vote for best synonym.