Here’s a short list of suggestions, based on different ways of thinking about “deadly”:
Kills the most people: the mosquito. I know that sounds bizarre, but some mosquitoes carry the disease malaria in their bites, and malaria kills two million people every year around the world. That’s one person every 16 seconds, on average. So just while you’ve been reading this, malaria from a mosquito bite has probably killed someone.
We don’t have malaria in the UK (so don’t worry if you get a mosquito bite!), and malaria could perhaps be eradicated worldwide if governments spent more money on research for vaccines, and on improving people’s water supplies (where the mosquitoes breed) in developing countries. But right now it is one of the world’s biggest killers.
Has the deadliest poison: there are several contenders for this title… but the winner is perhaps the box jellyfish – a single tentacle contains enough poison to kill 6000 people, and the poison works fast (most people who are stung while swimming never make it back to shore). Fortunately, they only live around northern Australia, and their stingers are quite small, so just wearing a thin suit of material like tights is enough to protect you. But I still wouldn’t swim through a swarm of them!
Most lethal predator: the biggest predator that lives on land is the polar bear, and I certainly wouldn’t wrestle one! In the sea, the great white shark is an awesome predator, of course, because like other sharks it is so superbly adapted to life as a hunter underwater. But it doesn’t actually have the largest teeth of any fish for its body size – that would be a deep-sea fish called the “fangtooth”. Fortunately, the fangtooth is nowhere near as big as a shark, so it is no threat to us.
Causes the most death and destruction: if we think about “deadly” as not just killing people, but other animals as well, then the world’s deadliest animal is… us! We destroy whole ecosystems, for example ripping up rainforest so we can graze cattle there instead, on a scale that no animal has ever done before in the history of our planet. And we have the power to devastate most of the world (at least above the waves) with nuclear weapons, if we were ever crazy enough to use them.