First of All, Joseph Morgan
Despite Klaus being a very cruel and ruthless character, many people have come to love him because of Morgan's brilliant acting. Morgan's ability to express emotions brings a welcome complexity to the character, making us feel for and empathize with Klaus, despite all the evil things he does. Without Morgan's raw acting talent, Klaus would just be another black-and-white villain. In fact, executive producer and writer Julie Plec admitted in an interview with TVLine that Klaus was intended to die in season three of The Vampire Diaries, but Morgan's performance is a big part of what changed their minds.