Simply from looking at my home airport here in Kansas City, it's obviously the Boeing 737 family. Kansas City has a little over 300 daily departures, on 18 airlines, to over 50 destinations in the US, and one in Canada. At KCI this is what the following airlines operate.......
United, Continental, Southwest, America West, Frontier, Delta Express, Vanguard, USAirways and occasionally American with their 738s (I talked to the AA station manager about the MD-80, 737-800 fleet plans and when we'd start seeing 738s and he said we already see them every now and then).
McDonnell Douglas MD-80/90/DC-9 series:
TWA, Delta, American, Midwest Express, Vanguard, USAirways
Delta, United, TWA (occasionally)
Delta, American, United
CRJ and ERJ:
Comair, Continental Express
Airbus A320 family:
USAirways, United, soon to come....Frontier
Air Midwest (USAirways Express)
Airborne Express, UPS, DHL
That's all I can think of at the moment!
BABY BOEINGS ALL THE WAY!!!
FLY DELTA JETS!