I'm thinking of getting a diskless Synology DS416play NAS to act primarily (for now) as a Plex server for my movies.
If Plex is going to be your primary use of the NAS You might want to consider the Western Digital MyCloud Pro Pr2100 and Pr4100:
https://www.amazon.com/Diskless-Clo...ie=UTF8&qid=1489706115&sr=1-1&keywords=pr4100
When they were introduced Plex sent out an email saying:
My Cloud Pro Series are transcoding and storage powerhouses
The (compensated) Amazon review is very impressive:
I ripped a handful of movies off my Blu Ray disks to MKV format, dropped them on the PR4100, setup Plex (SO EASY) and was streaming movies to test. I tested on an Apple TV with Plex, an iPhone 6s Plus, an iPad Pro, and a MacBook AT THE SAME TIME with four different movies, all transcoding and playing in real time at 1080p.
The Sinology is powered by a 1.6 GHz Intel Celeron N3060 Dual-Core while the PR4100 is powered by a 1.6 GHz Intel Pentium N3710 Quad-Core. One cpu website compared the cpus and called it a draw in terms of performance. Another rated the N3710 138% faster in quad-core. Guess it depends how Plex does multitasking and threading.
This may be is the best performing NAS for Plex.
As for disks I am not a fan of Western Digital as I have had a lot of failures. WD has been one of the worst performing drives on the BackBlaze disk failure reports. Here's the latest one:
https://www.backblaze.com/blog/hard-drive-benchmark-stats-2016/
I think they are no longer using them. Not sure of the reason.