Prime's Return. The turning point for Countdown
Countdown finally hits a bullseye with this issue. Superman Prime arrives on Earth 15 and kills that world's Superman brutally and then engages in battle with the rest of this Earth's heroes. Prime kidnaps the Earth 15 Lex Luthor and flys him up to the atmosphere killing him. He flies through Earth 15's core and blows it to kingdom come.
This issue was great. With Superman Prime finally proving just how dangerous and psychotic he really is, the entire Multiverse is in danger. And with Prime's new found power from his battle in the Sinestro Corps War, Countdown has just become fantastic.