I remember from the Gulf war in 1991, that the news said the amount of air sorties equalled (on a peak) the sorties being flown in WW2.
This being hairy and easy to misunderstand caused me to look into it a bit better.
What I found out from available sources was that the peak sorties days of the gulf war had the same level as an average day in WW2.
In sheer manhours, there is no warfare that matches WW2.
And Kursk is the way biggest tankbattle.
However, you'd be surprized how some things on the western front were big as well......