in reply to The most useful key on my keyboard is:
I voted for Control because I like to control things.
(Control is also very useful because you search buildings (other than the town center) with it in Age of Mythology.)
I also find Escape very useful because (in the fortunate case) you can cancel drags with it before you drop, and that's a very useful function. I don't understand why IDEs aren't made so that clicking the right button while still dragging something with the left button cancels the drag -- that combination never does anything useful currently.