In this case I would have thought the number of movements would be the best way to equate which is busiest
CDG has well over 500,000 per year but with 4 runways which are all basically parallel
FRA has somewhere near 450,000 from 3 runways
LHR 467,000 from 2 runways
CDG has one other busy airport nearby
Orly
Whilst LHR has 3 in LGW, STN and LTN
I cannot comment on North American airports
I did see fun and games once at MIA when one of the taxiways became congested making it impossible for an aircraft to clear the from the cross runway forcing go arounds off 2 runways and I would think a very high work load for the controllers
G-I-B