The SCD-1. Take a look at Audio Asylum on this, there are a lot of posts on this topic on the Hi-Rez Highway forum. The big difference is the transport, which was a cost-no-object job on the SCD-1 and, according to the posts I recall, a much better unit than the subsequent models. Sony pulled out the stops for its initial flagship, has not improved on it yet for two-channel playback, concentrating more on multichannel and cheaper mass-market models.
