I don’t want to serve any content with my Pi, I just want it to be a Plex Player. What is the best way to go on this? Obviously I’ll want the smallest OS possible, so things are relatively quick.
Rasplex IS just a Plex client. So, yes.
The PMP RP2 embedded works great, too.
However, PMP is severals months away from being anywhere as feature rich as Rasplex and this is the Rasplex section not the PMP section. 
Regards
I’ve tried both and hands down Rasplex is the better choice.