Best Media Player from Extras for MKV and ISO Playback?

Hi folks. Just got this setup tonight and was wondering what the best media player out of what is available from the add ins was.  I see Media Player, Plex, I missing another? My main interest is for playback of MKVs & DVD ISO (is ISO possble?).  Suggestions please!  Thanks very much.  Also wondering if DVD ISOs with full menus is supported.  Thanks.