Considering this is only a contest between the Covenant and the Gravemind...big bad Flood boss tops them all. Truth and Tartarus both were fairly interesting and overall capable villains, but nothing in Halo tops the Gravemind, both in terms of capability and awe inspiring terror. The Didact shouldn't even be on the list at all, because he's not a villain...and 343idiocy's attempts to make him so are nothing but a bunch of canon -blam!- bullshit -__- It doesn't even fit their [i][b][u]OWN[/u][/b][/i] canon for pete's sake...and the Derpdact isn't even a good villain, he's just your generic, raging omnicidal, insane, moronic Saturday morning cartoon villain obsessed with killing every single human being whilst cackling maniacally and twirling his thick, greasy, black mustache.