Imperiex; destroyer of worlds, the embodiment of entropy, creator of the Big Bang. And yet, if you've only read comics made after 2006, you have literally no way of knowing about him, as the last time we saw him was in the 52 series. Even then, the devourer of souls is featured once, in the single smallest panel that human hands could possibly draw, just to emphasise a little extra how completely ignored the character is.
Admittedly, though, unlike many other powerful characters, there are some pretty justifiable reasons for Imperiex being mostly underused in comics - namely, that he is just a tiny bit dead, after the events of Action Comics #782.
But in the world of comics, this is a pretty weak excuse, and thus it seems far more likely that no DC writer wants to try and finagle another way for the Justice League to beat Imperiex, who by all means should be immortal.