Professor Zoom- The Flash: "The Death of Iris Allen"
Professor Zoom's knowledge of The Flash's secret identity allows him to make personal attacks on his enemy in much the same way that Green Goblin is able to attack Peter Parker. This idea of attacking the man behind the mask is evident in the "Death of Iris Allen" story arc which spans The Flash #275-284.
In this story, Reverse Flash murders Iris Allen by vibrating his hand through her head at a costume party. The Flash is devastated by the loss and goes sort of crazy. He asks Hal Jordan (Green Lantern) to use his power to resurrect Iris and considers giving up his mantle. It is this event that would, later, cause his downfall.