What is a vampires worst enemy?

werewolves, by far, since they were created by two clans who were at war with each other.

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It depends. There are many different types of vampires with different weaknesses and different enemies. Sometimes it's werewolves (for popular fantasy vampire) , sometimes it's God or men of faith (for the traditional satanic vampire), sometimes it's just humans in general (for the "modern" vampire).

Personally, I think that a vampire's worst enemy is [permanent] death, since the traditional excuse for becoming a vampire is a fear of dying/Final Judgment and because the enemies listed above are a vampire's enemies because they can kill them.