Eobard came back every season, Zoom actually became a reaper, Savitar's existence is the most confusing time paradox ever, don't remember Ricardo's death that well. Whale is dead? Did I not finish Black Lightning? Adrian's death blew up the island. Hmm... Hard to pick. Guess I'd go with Savitar because of the irony of being killed by the woman he was supposed to kill, and he just shouldn't have existed anyway.