Topic: Four Freshmen
The best vocal harmony groups I've ever heard are The Four Freshmen and Manhattan Transfer.
Most groups have the lead voice in the middle chord. The Freshmen were the first to put their lead voice at the top of the chord. This leads to arrangements that are as distinctive as fingerprints.
If you've ever heard a Four Freshmen track, you'd recognize another... anytime, anywhere. They also have unique musical phrasing, more like instruments would play than voices would sing.
The other thing that makes The Four Freshmen unique is their instrumental musical ability. Most singing groups just sing. The Freshmen provide their own accompaniment, playing nearly a dozen instruments.