We have no vipers, we have elapids, instead (mostly they have a small head). Think King Brown Snake, Cobra and Taipan. We have a thing called a 'Death Adder', no relation to the African 'Puff Adder', that is really an elapid, but with a triangular head.
The King Brown is the deadliest, not because of the venom, that goes to the Taipan, but because it hangs on and chews, injecting lots of venom.
A sea snake, Hydrophis belcheri, actually has the most toxic venom, but is very timid.
As for aggressive, the prize would probably go to the small Brown Tree Snake, which will look at you, and rather than run away, will then go for you. Fortunately, it is a colubrid, or rear-fanged snake, which is why I am still here...