What is the busiest non-domestic route of Turkish Airlines:
- in terms of passenger number,
- in terms of number of flights performed per year?
Is there a database that I can look for such kind of statistics for all other European airlines?
According to Turkish Airlines VP Ziya Taskent, they are adding a ninth weekly flight between Tel Aviv and Istanbul. He mentioned that:
695,000 [passengers] flew roundtrip with the airline in 2014 [between Tel Aviv and Istanbul], 4.86% more than in 2013, making it the leading foreign airline in Israel
The largest number of airports served by the carrier is Germany with 14 destinations.
For other European carriers as requested, the Wikipedia article 'List of busiest passenger air routes' is a good starting point.