window seat never is the best. Actually it depends on which airline you are travelling with. However as i just said, no window seat is the best except for the business or first class.
Depending on which airlines A340-600 you are flying on...
For Cathay Pacific, seat 30A/30K would be best as they are both window seat and located on the first row of their Economy Class sector... For Cathay Pacific, their seat 54A/54K are emergency exit row... However, I am not sure if there would be a window right next to the exit on row 54...
But beware, that on the first row of each sector, you will have no spaces to store bags under the seat in front... And any bags will need to be stored on the over-head locker...
Beside 30A/30K having the most legroom on window seats, there is no other seats which are special and having extra legroom... As all Economy Class are the same... But I would advice you not to choose any window seat from row 62 and onwards and that is where the aircraft get more and more narrow, which means when you watch the PTV, you will have to watch sideways... And on those seats from row 62 and onwards, there would be a bigger gap between the seat and the window compare to those in front...
So maybe you first choice could be 30A/30K, if those seats are not avaliable... Then any seat A or K from 31 to 61 would be the same... But from row 62 and onwards, they are a little uncomfort...