I'm only interested in NOS (non-oversampling R2R) DACs these days. I have 4 of them: 2 are always in use and 2 are always stored at backups. The best of these are the Metrum Onyx & MHDT Labs Orchid.
Those 2 DACs handily illustrate a key point about NOS & R2R DACs: when you select this technology, you still face the intimidating choice of DACs that err on the side of "neutral" (ie, not warm, very resolving, big/detailed soundstage) or "colorful" (ie, maybe somewhat warm, spacious and resonant soundstage). The Onyx is the former and the Orchid is the latter. I like both sounds, depending on the downstream gear used.
+1 to the suggestion of using a DDC (digital to digital converter) with any DAC you end up with. I have two systems, and a Matrix X SPDIF 3 DDC is in each. I found years ago that I preferred digital coax to USB; since then I got several terrific coax cables and am very happy with that sound. I've also played around with AES/BSU cables and like that sound. None of my NOS DACs have I2S inputs, but when I get a DAC that does, the DDC allows its use.