I answered Von Carsteins ages ago, due to preferring 'generic' options in general. I love that classic vampire feel. Plus the Von Carsteins are the most diverse, you can have your brutal warriors, refined aristocrats, and recondite arcanists running shoulder to shoulder all within a single bloodline.
Of course, that love of diversity led me to greatly prefer the post-bloodline approach to vamp counts. I like running Mannfred alongside a strigoi ghoul king lacky, or a Lhamian Vampiress with a Blood Dragon suitor, and so on. Even in this context, though, I still preferred the Von Carsteins for their classic vampire theme and their important place in Warhammer vampire lore.
The End Times have only encouraged this preference, with Vlad, Mannfred, and Isabella proving to be some of the most important, most interesting, and most dynamic players on the field.
So yeah, Von Carstein. It was my favorite before, and its only more so now.
That said, I personally would have preferred to see Zacharias (perhaps revealed to be W'soran after all, or maybe Nagash's call could have woken the hidden parasitic spirit of W'soran within Zacharias's mind) to Dieter or Luthor Harkon as one of Nagash's mortarchs. The Necrarchs were supposed to be the bloodline that stayed loyal to Nagash's ambitions. It's kind of criminal how little play they got in Nagash's return.