CPH-LUX might be one of the most profitable ones. It's CRJ200 (QI) or CRJ900 which are always filled up, almost no economy seats (economy extra or business only) and prices start at around 2200 DKK one-way.
Just took the flight a few days ago and it was 5000 DKK return ticket booked a few weeks in advance, last seats left.
People going to LUX have lots of money or work for the EU and the parliament pays (similar to BRU). And there's no competition....