Another pedant alert:

The Manhattan Transfer did very few songs a cappella. Their a cappella counterpart would be Singers Unlimited, whom they were undoubtedly modeling, and in fact most of MT's a cappella numbers were Gene Puerling arrangements.

Groups like The King's Singers and Chanticleer are certainly accomplished, but once you get into primarily classical groups, you're in a realm that doesn't quite fit the implicit limits of this venue, and one could list a plethora of equally accomplished groups in that realm.   For classical and Renaissance music, they can't be faulted, but their arrangements of folk and popular material are so drenched in "high art" and cutesy touches that you need to keep insulin handy. In my opinion, a "best" group rarely puts out an album you mostly cringe through.