Abaddon
Abaddon the Despoiler is Horus' successor and is renowned as the single greatest threat to the Imperium in the galaxy. This says more about the Imperium than Abaddon as his raging incompetence is now well known. He has launched thirteen consecutive Black Crusades against the Imperium, every one of which has failed miserably. He will not revise his strategy, so don't ask.
He also has a model in the tabletop game. After you watch this model go down in a single round to a couple of trigger-happy Tau Broadsides, it won't be fun anymore. Fucking railguns.
His 7th Black Crusade was a 'success' because he set his goal very very low this time, (He basically just fucked over the Blood Angels), but mostly it was to do with the fact he got an army of Khornate Berzerkers to fuck the Blood Angels over. Success is in quotes because even sending a whole crusade to target a single chapter is sad, especially when the chapter's still around at the end.
He also enjoys calling Horus a weakling and a fool, which is ironic, because Horus actually came close to killing the Emprah, wheras Abaddon has barely managed to take Cadia.