Ob Nixilis's Cruelty is without a doubt the strongest removal spell in the set. It costs only three mana and removes creatures at instant speed. On top of that, it requires only one black mana, which makes it splashable in Limited.
All this makes Ob Nixilis's Cruelty a definite pick in any kind of draft, since you can always salvage one spot for a black mana, which will be used for such a strong removal. And since this is at common rarity, you'll pick them in multiples.