The black mamba is much more aggressive therefore harder. Just being venomous doesn't make it hard on its own. If you want a cobra, i'd go for the spitting cobra as it spits venom in your eye just because it can......
I dispute that the mosquito actually kills, like the tsetse fly, it unintentionally infects you with a parasite which kills you which is different to venom. Not hard, case closed.
A left field suggestion, the shrew
Shrews distinguish themselves by their incredibly high metabolism, which gives them an extraordinarily high strung and vicious disposition. Shrews must eat constantly, and in order to sustain themselves, they savagely attack prey twice their own size...
The salt water croc
The world’s largest reptile also carries the most ferocious disposition. Saltwater Crocodiles reach lengths of over seven meters (23 ft) and may weigh up to 2,000 kg (4,400 lbs). Possessing immense jaws capable of the world’s most powerful bite, the full grown “Saltie” is...