Out of my head these are my Top 11 in no particular order: Toronto Pearson, Aerosoft’s Munich, Hamburg, Brussels, Nueremberg and Köln, FlyTampa’s Boston, Las Vegas, Edinburgh, Florence and Paris-Orly.
If you’re more into small GA airports I can highly recommend: Dundee, and everything from Burning Blue Designs and Pilot Plus.
One freeware airports that I really like is Duxford.