I recently researched this very point for a 9 hr daytime flight. You will find a mass of conflicting views!

We eventually chose the middle pair in row 4. I think much depends whether it is a daytime or a night time flight but more importantly, whether you like to chat to your other half.

Having now completed the flight, I would still choose the same in similar circumstances. I would say 2A and 2K are far too apart to chat; you might just about manage it in row 1.

Overall we had a great time - well looked after by our CC