I am a great fan of a capella singing. I was a member of The Mainliners (The Bryn Mawr chapter of The Society for the Preservation and Encouragement of Barbershop Quartet singing in America) and I, once, sang with The Persuasions at a swimming pool.
I have composed for choirs and taught music for forty years and I can say, without qualification, that TAKE 6 is the most astonishing vocal group I have ever heard. They have redefined Gospel and a capella and they have raised the bar forever. I took their first CD, "Spread Love" to a Mainliners rehearsal and blew everyone away.
Other dynamic a capella groups are Lambert, Hendricks and Ross and The Buffalo Bills (That's The Bills you hear in the movie, "The Music Man"). I kind of like the old Golden Ring, too.