What airline has the most diversified fleet? Cargo and passenger? I’d have to say FedEx.
It’s pretty tight between FedEx and UPS
Let’s see…UPS
B721s (for the next few months, anyway)
DC-8
B752
B767
B747
A300
MD-11
FedEx
B722
B767?
B752 (soon; taking delivery as we speak)
MD-11
B777F–orders placed
A310
DC-10
I’m sure I’m missing something…Feel free to chime in if you know.
As far as domestic passenger airlines go, my guess would be United (I’m not counting regionals d.b.a. “United Express,” etc.)
B733
B735
B752
B762 & 763
B772
B744
A319
A320
Overall pax airline w/ most diverse fleet: I’d say BA probably takes the cake; I’m not even going to try to list what they fly, but I will say that it would probably be easier to list what they DON’T fly.
Wow. I thought there’d be more. I guess the days of Fokkers, BAC-111s, and Tridents are long gone… 
Well it makes sense from a maintenance cost perspective, but it does take all the fun out of seeing so many different types of planes when you’re at LHR.
Cost-cutting measures (like RJs, for example) have always seemed to take some of the fun out of spotting.
Don’t forgot the gazillion Cessna 208’s FedEx has. They own them although they are operated by other carriers. FedEx also has a handful of F27’s and a few business jets.
NWA
AIRBUS
A319
A320
A330
BOEING
757
747
787 orders
CANADAIR
CRJ 200
CRJ 900
EMBARER
ERJ 170
i don’t understand why NWA didn’t go with the 767 or 777 (chose a330) and if they liked airbus, why wouldn’t they order the a350? They also chose to spilt e170 and crj900. That is high training costs. Nwa has management issues 
And the DC-9.
Fedex also flies A300, ATR 72, ATR 42
United Airlines/ United Express
Airbus A319
Airbus A320
Boeing 737-300
Boeing 737-500
Boeing 747-400
Boeing 757-200
Boeing 767-300
Boeing 777-200
Dash 8
CRJ 200
CRJ 700
Embraer 120
Embraer 145
Saab 340
I had the A300 on the list; must have accidentally deleted it. Didn’t have the ATRs though.
You left out the Saab 340, which is technically flown by Mesaba for NW Airlink. Pinnacle flies the regional jets for Northwest. If you include these “sattelite” airlines, then United flies more than the ones I listed above (as SWAflightattendant pointed out).
As far as the Airbus/Boeing orders, I think NWA lives in the moment…Whatever works for now is what they’ll go with. Like UAL, they originally ordered the A320 series aircraft when the 737s didn’t have the range needed for reliable transcontinental flights. By the time the 73Gs, 738s and -9s came around, both had sizeable Airbus fleets. Because of all the A32X orders, I’m willing to bet that Airbus gave them an attractive offer on the A330s, knowing that their fleet of DC-10s was on borrowed time as well. Hopefully for them, they’ll survive long enough to see the 787s… 