Niklaus Mikaelson, AKA Klaus, is known amongst us Vampire Diaries fans to be the strongest, baddest, vampire out there. He also is currently the main protagonist in the tv series The Originals.
Klaus is the strongest vampire of all time because he is known as a hybrid, half vampire half werewolf.
Klaus is the biological son of vikings Ansel and Esther Mikaelson, step-son of Mikael. He has three brothers, Finn, Elijah, and Kol. He has two sisters, their names are Freya and Rebekah. Mikael and Klaus throughout the whole tv series have a huge father son hatred, caused due to the fact that he is a bastard.
Why the baddest Vampire of all time was interested in Mystic Falls, the home of The Vampire Diaries?
Well, this answer has to do with the fact that Klaus was after breaking a curse that has been shed on him for over a thousand years now. In order to break this curse he needs a doppel ganger that looked like Kathryn Pierce. He is after this curse because he wants to be able to create more like him and ultimately his own species and army. (More about Klaus here... http://vampirediaries.wikia.com/wiki/Niklaus_Mikaelson)