The term "doomsday asteroid" is usually applied to an asteroid that, if it impacted Earth, would cause sufficient destruction to significantly disrupt the balance of life and possibly cause major extinctions. Such an asteroid would have to have an orbit that intersects the path of the Earth.
One of the likeliest candidates for the title of doomsday asteroid is 99942 Apophis. Apophis crosses Earth's orbit every 9 years, and has a mass of 20 million tons. According to CNN, the 1,000-foot-wide asteroid is set to pass close enough to graze Earth's satellites in 2029. In the event of an impact, Apophis would explode with the force of over 1 gigaton, or 1 billion tons, of TNT.