Typically, airline reserve flight number “1” for their flagship route. After obtaining several Heathrow slots from Air France, Delta inaugurated DL1 from JFK to LHR (London Heathrow). Recently I had the opportunity to travel on DL1. For me, the über aviation geek, it was quite enjoyable.
Other Notable flagship routes:
Continental Airlines, CO1: Houston (IAH) to Honolulu (HNL)
American Airlines, AA1: New York (JFK) to Los Angeles (LAX)
United Airlines, UA1: Chicago (ORD) to Honolulu (HNL)
US Airways, US1: Phoenix (PHX) to Chicago (ORD)
Turkish Airlines, TK1: Istanbul (IST) to New York (JFK)
Lufthansa, LH1: Hamburg (HAM) to Frankfurt (FRA)
Japan Airlines, JL1: San Francisco (SFO) to Tokyo (NRT)
Korean Airlines, KE1: Seoul (ICN) to Tokyo (NRT)