The best DAC I've heard so far is the one I am currently using built by Alex Peychev of APL Hifi - Denon 3910 chassis, 4 32-bit 1st-generation AKM Dacs, digital input, Class A tube output stage, etc.
I'm also fortunate in that it appears later this week I will finally receive the first APL stand-alone Super-DAC consisting of most of the mods done to create the amazing NWO-4.0SE, including 20 second-generation AKM 32-bit DAC chips per channel.
As good as the 3910 DAC is (and it is VERY good in my system) the new one promises to be substantially better. That's difficult for me to conceive of at this point, but Alex has never let me down, so I guess we'll see how it sounds in my system. He is well known for pushing the digital envelope in a good way, so I'm pretty excited.