I built a Plex headless server using a supermicro X9DLR-3F MB and two Xeon E5-2650 CPUs. This was years ago and I was wondering if anyone could subbest a better build that would substantially increase my ability to transcode media. This server is only used as a Plex server.
If all you are worried about is running Plex and getting the most/best possible transcoding. I believe that the most commonly recommended advice on this forum is to use a dedicated Plex transcode box. The options that are currently being recommended a lot are the HP 290, HP ProDesk 400 G4 SFF, or a DIY build that is equivalent.
Whichever way you decide to go you can read the following guide that describes how to setup a dedicated Plex transcode box to get the most out of the hardware.