I would go with the B seat. All the windows seats in Economy 777's on AA have the "Entertainment Box" under seats A and J, whcih basically blocks off about a third of the width of the seat infront of you for legroom.

For simialr reasons I'd avoid the D and F seats in the middle section, and go for the central E seat every time if you're not an Aisle person as they also have the entertainment boxes under them.