Which airport can I unlock that is good with eight gold planes that I can level up fast?
Which airports do you have already? I would recommend not wasting GPs on unlocking airports and rather unlock them by levelling through previous airports, that way you can save them for fleet planes and other stuff not available without using GPs. If you’re just beginning, SXM is good to unlock early to have another line of progress though (for only 4 GP)
Honestly I disagree. You can save months or years by spending two weeks of GP.
Depends on how much you play, some people play WoA as their primary game and spend multiple hours per day which makes progressing to unlock airport quick whereas other people like me only spend around 2 hours per day which makes unlocking with GP better.
Also, in answer to your question, I’m not sure what you can get for eight. I don’t remember the prices. However, the best airports in general are, in order: IAD, BKK, LHR, SCL, MCT, LEJ/SYD
This is opinion but I think most people will have a similar list. IAD has 4 active runways and BKK has three, so those are the most efficient. Most single runway airports will be pretty similar to each other. SXM, INN, and BRI are the worst by far due to low stand count or poor runway/taxi layout.