For hi-fi purposes, none.
Class B is often used for radiofrequency broadcast amplifiers, where high power and low spurious RF emissions (sorry, no class-D in radio!) make it a great choice. On its own though, class B suffers from relatively high (crossover) distortion and is unsuitable for audio for that reason.
Combined with a little class-A though, you get the best of both worlds. When a little quiescent current is applied to the output devices, they are biased on past the zero-cross point, and you then have a class AB amplifier. This dramatically reduces distortion and is what most proper consumer-grade hi-fi amplifiers use these days.
