How hot was the biggest heatwave on earth?

It's hard to say, because different countries have different definitions of what a heat wave is. One day of record high temperatures is not a heat wave. A heat wave lasts for several days.

Heat waves in 2014 reached 43C (110F) in Australia for over a week.
It's hard to say, because different countries have different definitions of what a heat wave is. One day of record high temperatures is not a heat wave. A heat wave lasts for several days.

Heat waves in 2014 reached 43C (110F) in Australia for over a week.