Quite conveniently, the EU has an entire department (Eurostat) computing statistics about anything and everything. It takes them a little while to process so much data though. And so, in 2010:
#1 Spain UK: 28.8 million passengers
#2 Germany Spain: 21.0 millions
#3 Germany UK: 11.1 millions
With that said, I am not sure such ranking is as significant in Europe. Air transport is by far the most common method to travel between Korea and Japan, but in Europe, in many cases, road or train travel is.
For example, by air, there were 10 million passengers between the UK and France in 2010. Just adding people who took Eurostar alone more than doubles that number. Throw in the ferries and Eurotunnel's car shuttle, and I am pretty sure that overall, there are more people traveling between France and the UK than any other two countries in Europe, just as I would think more people overall travel between the US and Mexico or Canada than between the UK and the US.
Recep Tayyip Erdoğan has no clothes.