It looks as though Air Berlin is the only operator, flying into TXL (Berlin-Tegel, same as BA, AF etc). IMO they are one of the better operators in Europe, better than most of the legacy airlines - with catering, seat numbers, FF program.

I made a dummy booking, something like £70 each way. Austrian are a bit more expensive, Lufthansa more so.

AB have sales from time to time but IMO it would be unwise to rely on these as they are short term and may not have the route you need. Best to book as soon as you are sure of your dates, remembering that lower fares are normally non-changeable.