I, myself own the Bizzare Murders, Ghosts, UFO Files, and the Strange Legends dvd sets. I'm not big into Miracles or Psychics, so I passed on those. Anyways, I was a bit confused about the new set released by First Look. From hearing people describe them, I thought that there was only one set as well. I just thought I'd clear it up for those who were having troubles like myself.
The Unsolved Mysteries: Ultimate Collection Treasure Chest is mainly for those who haven't purchased any of the individual sets. This Ultimate Collection I believe has all of the previous six dvd sets included as well as a new bonus set which includes new ghost stories as well as new treasure stories.
The Best of Unsolved Mysteries is basically for those who had already purchased all or at least their desired individual sets. This particular set has several new ghost stories on it as well as several new treasure stories. The rest is a mix of repeats from the individual sets as well as some new ones here and there.

Anyways, I understand that this will seem rather noobish to many of you, but I'm sure that it will help clear up things for a few people. Also, feel free to correct any of my information if it's wrong.

The Best Of Is stories from the 6previous sets, and they have New Ghost stories, and some other NEW stories that weren't on the other previos 6 sets.

^Like GGF92 said, The Best Of has most of the stories that were on the previous six box sets, as well as the same Bonus Treasures and Ghosts DVD that was included in 'The Ultimate Collection'. Also, some NEW ones that are marked NEW on the cases. Hope that helps.

Cheers