As seen here: tvshowtime.com
I know there are many TV trackers out there, and the Trakt.tv request is already in so I’m adding my vote for TVShowTime.
A 2-way sync would be preferred:
-Watching an episode in Plex would mark that episode as watched in TVShowTime. This is probably the easier one to achieve 
-Adding shows in TVShowTime would add them to a “to watch” list in Plex. From there they might be sourced from live TV, one of the channels, your own media, etc.
