I think row 6 is the best on the 764, because you are being served by the rear galley that only services rows 6 & 7, so the service is much more personalized.

I prefer row 6 to row 7 because you are one row removed from the galley and lav.

As a solo passenger, my favorite seat on the 764 is the solo middle aisle (6D).

If you're flying with someone then either one of the window-aisles combos is fine, although please note that the rear BF lav is on the right side of the aircraft, which would give a slight preference to 6A & B.