If there are seats facing each other, I tend to go for those if unoccupied. Otherwise, I'll go for the second row down from the back where the emergency door is located as that offers the most legroom.
On a decker, it has to be upstairs, backseat. The front seats, I find, are rather cramped and you end up sitting either sideways or at some other weird angle.
RE: Things you don't always think about....