"For the busiest class C my guess is PDX, MKE, or MDW and for the busiest class D my guess is TEB or VNY. "
Here's a list of airports with Class C airspace-
http://en.wikipedia.org/wiki/Class_C_Airports
Here's the Top 50 airports 'Ranked By Passenger Enplanments' in the U.S.-
http://www.bts.gov/publications/national_transportation_statistics/2004/html/table_01_41.html
Looks like
MDW is the busiest Class C based on enplanement criteria. Haven't located a 2004 ranking for airport operations to confirm on that criteria.
I'd also agree on
VNY because I have heard it's the busiest
GA airport, although again, I can't yet find a chart including
GA ops too so it can be confirmed statistically.
"Van Nuys Airport (
VNY) is ranked as the world's busiest general aviation airport.
VNY averages nearly one-half million takeoffs and landings annually, with 466,449 total operations in 2003."
http://www.lawa.org/vny/generalDescription.cfm
"For the least busy class B I'd have to guess MSY or MCI."
HOU is actually the lowest on the Top 50 list at #45 .
MSY is #38 &
MCI ranks #36.