I’ve read the first 3 volumes of The Flash. Never played Injustice.
I also recommend my personal favorite, Identity Crisis, one of the all time greatest comics ever made. Trust me.
Edit: Sorry to anyone who already posted, but I attempted to post an image of my options which clearly hasn’t worked.
Which of these should I read next?
Dark Nights: Metal
The Flash vol. 4: Reverse
Action Comics (2011) vol. 1: Superman and The Men of Steel
Injustice: Year One (The Complete Collection)
I’d go Dark Nights Metal. I like Scott Snyder & Greg Capullo. One of the best creative teams in all of comics.
I second Dark Nights: Metal.
Horns up, Metal it is
With Dark Knights: Metal, while it is written in a way that explains everything, it is something that builds on a lot of other works, so be prepared.
And while it is based on the game, the Injustice comic really goes more into how the world set up in the series came to where it is in the game. In fact, they would later adapt the events of the game as a comic series called Ground Zero, where it basically tells the story of the game from Harley Quinn’s perspective. Or if you don’t want to do that, you can find videos on YouTube that show the full story mode of both games.