On the 757, 8A or 8C would be preferred. these are exit row seats with lots of room and no one in front of you (except FAs in the jump seat during takeoff/landing.

8F has room but you have a row of twin seats, 7D & 7F, on the side and in front of you. These duo seats are also good (though lack the leg room of 8A-C).

The window seats tend to be cool though due to air leaks.