Carmilla

I like this story a lot. It comes from a time before vampires were in every single piece of fiction. Luckily, it's not so old that the language is unreadable, a fate that befell Polidori's The Vampyre. I haven't gotten around to Dracula yet, so I can't make the same comparisons I see others make, but I can say that I think this is beautiful. The language is gripping, and the emphasis that Le Fanu places on the mysticism and seductive powers of the vampire makes the genre feel unique from other styles of horror.

Side note: I believe this is the oldest story I've ever read with a strong LGBT presence in it. That on its own makes it noteworthy.