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I use MPlayerX, have never encountered a video file that could not be played wonderfully by it.
 
I use Plex for all my movies, TV shows and music videos. It's great for managing content, automatically downloading movie posters, subtitles and metadata. It plays content from multiple drives on the network and plays on any computer, HDTV or iPhone selected. I can control it remotely with the iPhone app. It also handles more file types than iTunes.
 
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