Originally Posted by: LeedoggWhat do you mean by strongest? I'm not trying to be a dick, I just want a clarification.

Well because all three notes of the triad are different. C major contains the notes C E G, while D minor has D F A. So if were to go from C major to D minor in a progression, the change would be stronger. All other movements contain at least one similar note, sometimes more.

unison = 3 similar notes
thirds = 2 similar notes
fifths = 1 similar note
seconds = none