Plex sets by default on each device the video quality to 720p, this means that your video quality each time you watch something will be transcoded down from 1080p to 720p, by doing this you will lose the quality from the image.
You are able to use the following settings from below on each device in order to stream at the original quality(highest) of the content and get the best video quality out of the server.
These settings from below will help get every pixel from the server to your TV or screen that is meant to be seen.