I don't know what Delta's most profitable route is, but I'd say their "bread and butter" is Europe, along with Continental.
Delta serves the following destinations in Europe:
Continental serves the following:
Obviously both airlines have a huge presence in Europe, notably serving some destinations that have no other longhaul service or are not served by any US airline, such as Edinburgh, Istanbul, Moscow, Barcelona, Lisbon, Geneva, Oslo, Athens, Venice, Stuttgart, Nice, and Birmingham. All of those fit one or both of those two categories.