First, guns do not kill people - they are inanimate objects. Probably the most common firearm, and due to commonality, likely to be used in more shootings, is the .22 rimfire.
Overall, based on sheer numbers produced and the intended audience, the AK family of automatic assault rifles likely kills the most people world-wide. The AK family is generally chambered for one of two calibers: the 7.62x39 or 5.45x39, with the 7.62x39 being far more common (about 10:1 in numbers).
If one is talking only about murders and other deaths in a civilian setting (not wartime or other settings), then the most common would likely be one of: 7.62x39 (AK family), 9mmx19 Parabellum (most handguns), or the .40 S&W (majority of police handguns). Note that, world-wide, suicides and gun accidents make up the majority of gun-related deaths in a civilian setting, with murders and police shootings coming in well below either of them.
Train
106 people each year
Approximately 6 to 7 million people walk on Charles Bridge each year, making it one of the most popular attractions in Prague.
Florida is the US county that receives the most lightning deaths each year. On average, about 10 people die from lightning strikes in Florida annually.
Approximately 13 million people visit Woodbury Common Premium Outlets each year, making it one of the most popular shopping destinations in the world.
House fly
That's a very difficult question to answer because, of course, any caliber can kill. Smaller caliber handguns are fairly common in crime involving handguns because people believe large handguns are too diffucult to conceal. Most major police agencies in the U.S. use 9mm or .40 caliber handguns. It's also difficult to answer because information on caliber used in homicide and other crimes isn't readily available.
Pneumonic Plague, if not treated within the first 24 - 48 hours of infection, the mortality rate is 100%. If it is, the mortality rate is 75%.
the mosquitoe
cancer
I can say that i shot a fine Antelope buck with my 270Win caliber rifle at 403 paces in Wyoming.I have seen this caliber make many kills from that distance on in to 35 yards.It all depends on the game animal hunted,and most important your rifle skills.
Overall, lighitng kills the most people, though tornadoes are not far behind.
the thing is there is a different amount every year so there is no real answer
the most harmful disease in the world is cancer (it kills the most people).
is it cocaine or tobacco The answer above is incorrect, as cocaine is not legal and tobacco is not a drug. Alcohol is most likely the answer to your question, it is legal and also kills thousands of people each year, some of which were not even using the drug... but were victims of the effects the alcohol had on the person who killed them.
Love
Most people killed in tornadoes are struck by or crushed under flying or falling debris.