I've been in BNA about a dozen times in the past year. Here's my history of rentals since we switched to National in Sept (as memory permits):

Today:
Dodge Charger (alternatives: Impala, Town&Country, SantaFe)

Last week:
Chrysler 300 Touring (alternatives: Impala, Malibu, Town&Country)

Other visits:
Chevy Impala (multiple times - sigh)
Chevy Malibu (Nov 09)
Ford Edge ^ (Jan 10)
Dodge Avenger (Jan 10)

In the EA I have regularly seen:

Chevy Impalas O'Plenty
Ford Fusion
Chrysler Town & Country
Mercury Grand Marquis
Chevy Malibu
Toyota Camry
Pontiac G6
Chrysler Sebring

Generally nothing special. As ElmhurstNick eluded, the "back end" of the lot as you exit has some of the nicer cars. I've seen the same Volvo C70 (hardtop conv) parked in pretty much the same spot since March. The bigger SUVs (Tahoes) are back that way.

I've had some poor timing where the person ahead of me got a Nissan 370Z or a Chevy Tahoe. I can't believe they'd pass an Impala.