My favorite is San Francisco, bayshore hwy runs right along the bay near both sets of runways.There are awesome, and famous views of the staggered simultaneous landings they do there, as well as a large variety of international traffic. With a 300mm lens you can capture quite a few great shots. Here's a pic(not mine) from that spot:
Among the best places to have gone on a "non-official" session, but having the opportunity to spot various airlines from all corners of the Earth would be none other than Los Angeles International (LAX).
A classic example of the many airlines you could see in just one glance!