In terms of variety of airlines i think the UK is number one. There are many countries with direct service to the UK. Especially at Heathrow where I would consider "rare" airlines (airlines that are unique outside their country or region, or without a huge global network) such as Air Seychelles, Air Mauritius, Uzbekistan Airways, Royal Brunei, Ghana Airways, Syrianair, Air Zimbabwe, Air New Zealand operate. Again, it is very subjective as mentioned. To someone in Australia, Air New Zealand, Royal Brunei or Garuda are common, but are a treat in the UK.