Oooh, a fun question!
Deadliest is the box jellyfish – touching it it will straight up kill you.
Scariest is totally subjective to whomever you’re asking, and what they’re afraid of. Some people are really scared of big predators like tigers, but I’ve spent enough time around them that while I have a healthy amount of caution in their presence and I’m very aware they can kill me easily, I’m not scared of them, ya dig?
Honestly, for me, it’s parrots. I can’t read their behavior, which makes me uneasy around them, and they’ve got a sense of humor and ability to be tempestuous that I don’t like combined with the ability to crush fingers in their beaks and a minimal amount of body language I find discernible.
Statistically, the deadliest animal is the mosquito, not for its own abilities but for the pathogens that can breed in its body.
Deadliest for its own ability would be the box jellyfish or a similar jellyfish, since coming into full contact with one will probably kill you very quickly.