HGST Deskstar NAS.
Reds are probably too slow, the Red Pro is unproven, and the RE4 is a noisy, hot running beast. The Seagate NAS is just a relabelled Desktop.15, so same issue as the Red as far as speed is concerned.
Though, to be fair the 412+ is pretty old now, there's faster NAS units out there for probably less money. The 415play has hardware encoding for certain apps, I'm not sure if that includes Plex just yet.
I'd recommend the Asustor AS604T. It is an Atom based unit (D2700), 2 cores, 4 threads, and totally wipes the floor with most of the Synology units. I'm also finding the Asustor is more stable than Synology - with the various Synos I had, I was always having to tinker and fiddle with them to keep everything happy. The Asustor I plugged in, copied the files over, and basically forgotten about it since. Even an AS204T is probably as fast as the 412+, though it lacks expansion options (the 604T has 2x eSATA expansion ports).