From this three the best choice geographically is SYD.
Few reasons:
1- as was sayed by devs is best connection between Asia,Europe,Africa and Americas
2- big amount of all stands
3- almost at the limit range of AN-225 ( the newest plane )
Next three are my best choice for NOT too 3 by votes airports and all of then are in Asia region.
As follow by highest desire ti be playable:
KUL - Malaysia
DEL - India
ICN - South Korea
All three are big international hubs geographically friendly with fast connections between the continents.
All three HAVE posibility to accept AN-225 and to send it to IAD and NGO
All three have good amount of M,L and X stands
Bottom line:
Choice what is best for the dynamic of the game,to not become boring after 1 month playing,vote/choice not by your country of origin!
Good luck and I will be happy to see one very new and interesting playable airport : )