Dumbledore was the only wizard that scared Bellatrix. One of the main reasons behind this fear was that Dumbledore was known to be one of the most powerful wizards, and he had the Elder Wand. Despite this, Bellatrix was the only Death Eater who managed to block a spell from Dumbledore. That's why she was highly ranked in Voldemort's Army, as she was able to fight one of the greatest wizards of all time.