I would say for airlines (maybe aircraft, but I can’t fully say) is a quite surprising answer, and it would probably be Portland International (KPDX). The reason being is it’s Boeings paint facility. Or at least one of them.
We see a huge variety of airlines here. Not all officially operating with passengers, but we see them here. Practically all airlines that operate Boeing aircraft except some of the 787s and aircraft manufactured elsewhere have flown out of PDX.
For instance, I saw an Ethiopian Airlines 737 MAX the other day, a Norwegian 787, etc, etc.
I’d think Davis-Monthan Air Base’s 309th AMARG, Mojave Air and Space Port or Pinal Airpark would have both the largest diversity of airlines and aircraft.
DFW. just kidding every plane is American Airlines in all actuality DFW serves all sorts of stuff from PC-12 C208 and King Airs to the 777 747 and the British Airways A380 if you want a diverse spotting trip then just spend one day at DFW
Singapore Changi? Spot here and we get alot of variety of passenger airlines and cargo airlines same goes for aircraft we have both narrow body and wide body