Everyone loves Lucy. Seriously: Quincey Morris, Jack Seward, and Arthur Holmwood all propose to her, Van Helsing thinks she's the sweetest thing ever, and even Mina can't stop talking about how gorgeous Lucy is. Lucy's like a child: she's blonde and innocent and seems very vulnerable, which inspires everyone around her to protect her. But this raises some questions: why is Lucy so vulnerable? What makes Dracula choose her as his first victim in England?
One possible answer is Lucy's natural sexiness. She's innocent and virginal, but she's naturally much more sexy and voluptuous than Mina is. If Mina is everyone's mother, Lucy is everyone's wife: at one point, she says to Mina, "Why can't they let a girl marry three men, or as many as want her, and save all this trouble?" (5.11). Though Lucy wouldn't mind marrying all three of the men who propose to her, she chooses Arthur, because she loves him best.
Later, Lucy's wish sort of comes true. After Lucy dies, Arthur says that he feels like he was really married to her by sharing his blood with her in a blood transfusion. Of course, he doesn't realize that Jack Seward, Quincey Morris, and Van Helsing all gave her blood transfusions, too. So if sharing blood with Lucy means being married to Lucy, Lucy got her wish after all, as Van Helsing remarks later to Dr. Seward (13.85).
And as a vampire, Lucy's natural physical attractiveness comes out: her "purity" is turned "to voluptuous wantonness" (16.17). Although she was never as maternal as Mina, any maternal instincts Lucy had before she was a vampire are perverted – she clutches a child to her chest, but instead of feeding the child, she feeds on it. And when she is interrupted by the men in the graveyard, she "fl[ings] [the child] to the ground, callous as a devil" (16.19).
There are other possible explanations as to why Dracula attacks Lucy, and why she's more vulnerable than anyone else. Perhaps it's a result of her passivity, rather than her natural voluptuousness. After all, she's easier for Dracula to influence and control when she's passively sleepwalking than when she's conscious and awake. Why do you think Dracula goes for Lucy? What makes her so appealing to the vampire?