i guess the choice is now eurowings or condor. both flying from geman cities but connecting within europe is easy enough.
there are also some charter flights.
I've personally always avoided AirBerlin due to their pricing... I've always found them significantly more expensive with less service than Condor and Eurowings...
We've used both, Condor and Eurowings, Condor from Frankfurt and Eurowings from Cologne.
Nothing bad to say about either airline, somehow I have always felt more secure flying Boeing aircrafts, so we have mostly used Condor, but with Eurowings, on occasion, you get more luggage with you than Condor.