This could be a new section: For each source (video/photo/music) you can display the favorites like you do with “most recently added”, …
You can already do that…
Create a smart collection based on an advanced filter (e.g. w/ rating > 4 stars and whatever other criteria you want to apply). Then publish that smart collection to your library “recommended” tab or the server home page.
https://support.plex.tv/articles/publishing-collections/
Edit:
As for the suggestion title… having a “favorite” button has already been requested in a separate thread.
2021 clean-up: partly implemented, partly duplicate(see above)
