Besides the king, which you must protect, which piece do you most like to use as a tactical 'weapon' against your opponent? Keep in mind that pawns can progress towards become 'queen' and the queen herself is powerful, but can be easily taken advantage of if placed wrongfully.
I voted that my favorite piece is the knight. I think the knights have to most maneuverability and the best chance for surprise and opponent manipulative moves. On top of that, knights make an enticing take and thus end up usually being sacrificed by me, but not before doing allot of damage. I would have to completely rework my game strategy if I wasn't given the knight to work with.
My favorite piece, even though it is not actually as useful as others is the Bishop. I find that the angle that it moves is not always as effective but it is often overlooked by the opponent allowing it to move into positions that other pieces just simply couldn't get to.