Humans for sure,i mean look at them at season 6,they already pose the biggest threat vampires ever met!they are so many and they are fanatics against vampires!humans can be the most dangerous creatures when they are obsessed with hatred and when under fanatism!they will do far more bad and hideous things to vampires than other creatures ever did!

Witches for sure have been the biggest threat, but there are millions of humans and the faeries can blast them to other dimensions but... Witches