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According to wikipedia:

Flight number 1 is often used for an airline's "flagship" service. For example, British Airways flight 1 was the early morning supersonic Concorde service from London to New York City and is now a premium business-class only flight between the same cities; Air New Zealand flight 1 is the daily service from London to Auckland via Los Angeles; Qantas flight 1 is the daily Kangaroo Route from Sydney via Singapore to London. American Airlines Flight 1 is the daily flight from New York to Los Angeles; Both Singapore Airlines (SQ) and United Airlines (UA) Flights 1 are the daily flight from San Francisco to Singapore, although SQ1 does a stop-over in Hong Kong while UA1 is a nonstop flight, Virgin Atlantic VS1 is the daily Heathrow to Newark flight; and El Al flight 1 is the daily overnight service from Tel Aviv to New York City.