Why? Enjoyment is subjective. The plot is not.
Okay, since there are differing opinions on back stories, let me explain how I view this first so you all can adjust your recommendation.
Most characters are not aware of Manhattan's existence. The ones who do only saw glimpses and don't know his name. In this case, Watchmen as Manhattan's back story is only relevant for Doomsday Clock. I already read Watchmen and Doomsday Clock is on the list, so this one's a non-issue.
Next, these are what I know about the other back issues and their relevance to current story arcs.
Darkseid War is relevant to Wonder Woman, who is fighting Grail, Darkseid, and finding out about her brother.
Death and Return of Superman provides context to Doomsday and The Anti Superman Team.
Flashpoint is relevant to The Button and Flash War.
Now since the story I want to prioritize is The Manhattan Mystery, we cross reference The Manhattan Effect List everybody else already contribute with The Back Stories List.
Wonder Woman hasn't been connected with Manhattan, so for now her book and Darkseid War are out... or... in, since Batman was talking about the Mobius chair, but I already know what that is, so for me it's out.
Batman and The Flash have been affected by Manhattan, so their series and Flashpoint are in.
I can see the list changing once I actually read the books or when I get new info, but for now that's how we're going to narrow it down.