The answer to that question may surprise you. Bill Gates posted the infographic below, compiled from data from the World Health Organization and others. As it turns out, the animals we usually fear the most (sharks, lions) are nowhere near the most deadly.

Credit: GatesNotes
Yes, it turns out that mosquitoes are our worst enemies -- even worse than other humans. Gates writes:
What makes mosquitoes so dangerous? Despite their innocuous-sounding name-Spanish for "little fly"-they carry devastating diseases. The worst is malaria, which kills more than 600,000 people every year; another 200 million cases incapacitate people for days at a time. It threatens half of the world's population and causes billions of dollars in lost productivity annually. Other mosquito-borne diseases include dengue fever, yellow fever, and encephalitis.
Gates used this as a jumping-off point to introduce Mosquito Week, a campaign to raise awareness for mosquito-borne diseases and parasites: