7. The Flash - Rebirth
Many fans of Wally West were initially upset by Flash: Rebirth because Barry Allen returned from the Speed Force after his 'death', and they believed he might displace the current Flash. They were 100 percent correct. Only a couple of years after Rebirth came Flashpoint and the New 52, deleting Wally. Fans worried that Barry's return would mean Wally might not be important enough, so DC decided to address their concerns by erasing him from existence.
However, Rebirth has more importance beyond the first step in reversing the dynamic of the speedsters. It set up that Barry's greatest rival, Eobard Thawne, is the man who murdered his mother and led to his father being sent to jail for the crime. It becomes Barry's primary motivation in being the Flash and thus has had widespread effects in the DC continuity.
Because Reverse Flash murdered his mother, Barry altered the timeline in Flashpoint, which led to the rest of the universe and the New 52. The Flash has not only been defined by this series, but it has shaped the rest of DC's canon on multiple levels.