**Completely unscientific warning ***

ORD 96 columns in the UA September 2009 schedule
DEN 74
SFO 45
IAD 44
LAX 36

lots of reasons the schedule is not the best measure -- code shares/duplicated/discontinued flights/day of week different schedules/ ... but assuming all hubs are equally affected, this is an indicator.
I will leave it to others with more time on their hands to go the next level.

Definition of biggest??
-- most flights (OP's measure -- but is a 747 equal to an CRJ200)
-- most passengers
-- most gates
-- ???