Well, I would say
ATL, but I'd probably have to go w/ a mid-sized to smaller-sized airport like
CMH.
The reason is that at
CMH you get a decent blend of 752s, multiple 737 series, A319s and 320s, DC9s, ERJs, CRJs, MD80s, but all from several different airlines.
If I were to spot only at
ATL, I'd get the occasional 747 and 777 or two, as well as an A340 sighting even now and then, but everything else in between would mainly be
DL after
DL after
DL after
DL of mostly 737s, MD80s, 757s, etc.
"The most terrifying words in the Engligh language are, 'I'm from the government and I'm here to help.'"-Ronald Reagan