A lot of natural history museums used to contain a small box with the label 'The world's most dangerous animal'. if you looked inside the box you would find ... a mirror. Humans are by far the most dangerous creature that has ever existed; working together, using our brains, we are capable of overcoming any other creature.

Of other animals, it depends exactly what you mean by dangerous; but the creature responsible for the largest number of human deaths is the mosquito, which kills two million people a year in spite of all that modern medicine and technology can do. In fact, by this measure (number of deaths caused) most of the deadliest animals use poison of some kind (examples include certain species of snake and jellyfish.)

Figures can be misleading and people tend to look at this topic in different ways, but if you put 'world's most dangerous animal' in a search engine you will find some interesting lists - this one seems well researched and gives a lot of information.