Plex or Infuse. MrMC is a good one too, but has a slightly steeper UI learning curve, and the Siri Remote touchpad can be a tad sensitive, for example I'm constantly accidentally swiping ahead 10 minutes into a video instead of 10 seconds. Personally, I've found there isn't a 1-fits-all media library app. For my use, Plex is a foundation with Infuse and MrMC supplementing it -- primarily use the latter 2 for more comprehensive subtitle lookup/download.
The free version of Plex is very capable, and the only one that also works on each of my Roku, SmartTV, Windows 10, macOS, tvOS, and iOS devices. The paid version adds some nice functionality, but it's perfectly usable without a PlexPass subscription.