The best is such a subjective term.
The Delphina (lite) has often been considered the best because of it's DSP capability and MPEG offload drivers.
Concierto for the PIV has long been heralded as the worst as it's little more than a DAC. But I like mine.
The cheapest, with the best quality output are without a doubt the PCI sound cards, Soundblaster Live!
However as Mediator DMA is not handled in OS4.0 who knows what the RTA drivers there are??