Best Server To Run Roon Core

I doubt you'd notice a difference in SQ among the major choices**.
It comes down more to price, ease of integration and use, along with what else (if anything) you might want to do with the machine besides run Roon.

Options include:
* raspberry pi (probably the cheapest option; many users seem to be quite satisfied with them; involves a little DIY setup)
* Nucleus (a little pricey, but probably the easiest turn-key solution for running Roon Core)
* Nucleus alternatives such as the sonicTransporter, which tend to be a little cheaper than the Nucleus, possibly with faster processors, plus support for a few music-related applications besides Roon Core (such as HQ Player)
* Mac Mini (possibly a good choice if you might want a full-featured computer for tasks unrelated to Roon; compared to a Roon Nucleus, you'll probably pay less to get more, including faster processing, but this choice goes against some recommendations to dedicate a computer only to Roon processing)

** BACCH-SP not only can be configured to run as a Roon Core or endpoint, but also applies sophisticated "audiophile 3D" processes that do affect the SQ (soundstage/imaging) significantly. Very expensive, though.  

BACCH-SP appears to have preamp and dac capability as well. It is intriguing but I would be interested in how well it does all the things that it is designed to do

The main thing BACCH is designed to do is crosstalk cancellation for "3D" effects on soundstage and imaging. For that, you don’t need to spend $20K+ on a BACCH-SP device. You can get the BACCH4Mac version ($980-$6980). Theoretica offers a pre-configured Mac Mini, which you also can use to run Roon Core.