For looks, I would say either AA at DFW or DL at ATL.
When taxiing to and from certain rwys(forgot which ones) you see a few specks of Delta, and then you sea a never endless number of silverbirds.
With DL at ATL, it is amazing to go around all of the terminals and look at just about every plane Delta operates(it seems that way).
For easiness, I would say AA to BA at LHR, you have to switch terminals, but it is an easy switch and on the way, you can see a nice variety of airlines and types.
My absolute favorite is American at O'Hare. Connecting there is very easy, even when connecting to and from an international flight. It is really neat to see all of the American, United, and foreign airline's heavies.
If you are there, make sure to check out the old Admirals club between Councourse K and I believe L. You can essentially see the bulk of American's domestic ops, and the international terminal with the heavies.
Give me a break, I created this username when I was a kid...