Bill Tries to Kill Eric (Season 3, Episode 12), Eric Tries to Kill Bill (Season 4, Episode 9)
"All's fair in love and war" came to define the power struggle between the show's primary vampires, which was further complicated by their shared love of Sookie. Bill got the girl, while Eric got the power – until the end of Season Three. While on a mission for the queen to kill the king of Mississippi, Bill saw his opportunity to exact revenge on Eric, and shoved his sheriff into the cement-filled construction site after the king. Eric then got his revenge in two parts: first, by surviving and revealing to Sookie that Bill had been employed by the vampire queen of Louisiana all along, despite his claims of genuinely loving her, and second by unintentionally attempting to kill Bill at the end of Season Four (unintentionally only because he was under Marnie's curse). When season five started, the two were united because neither had the power nor the girl – and it's only a matter of time before that changes again.