Earthquakes are caused when two blocks or plates of earth slip each other usually on fault lines which are spread all over the earth. This causes a shock which can vary from few seconds to minutes.
The largest and biggest earth quake which killed the greatest number of people is known to be the earthquake which came on December 26 2004 in Northern Sumatra, Indonesia. The earthquake triggered a huge tsunami which affected 14 countries and killed more than 230,000 people.
But if we rank earthquake according to their magnitudes then this was not the worst, rather there were earthquakes which were worse but killed far fewer people because the area was not densely populated and no tsunami was triggered.
Top 5 earthquakes according to magnitude are following:
1) Southern Chile Earthquake with 9.5 Mw
2) Alaska Earthquake with 9.1 Mw
3) Northern Sumatra, Indonesia Earthquake of 9.1 Mw
4) Northern Coast, Japan Earthquake with 9 Mw
5) Kamchatka, Russia Earthquake of 8.9 Mw