Of the 3, I'm only really familiar with the Challs, having read about half the Kirby stories in various formats before. Adam Strange: I've read only a few issues. No Manhunter tales beyond the origin and assorted panels illustrating the "power of the week" absurdities that everyone is posting from the Showcase.
So, I would recommend the Challs: the art is very nice on the Kirby side and lots there for fans of Kirby, the FF, Incredibles, or Darwyn Cooke's New Frontier.
On the other hand, Mystery in Space is supposedly a cornerstone of DC's Silver Age revival and Infantino's art is unique, to say the least.
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some reflections on the Showcase slowdown
http://www.comicsreporter.com/index.php/the_cr_misinformation_department/