From a recent interview:
< Which is your favorite piece on the board and why?Topalov: All of them. They are just like my children, and I take care of them. I haven't got a favorite piece. >
This may be true, but I've noticed lately that Topalov seems to take better care of his bishops than his rooks. Here are some interesting games where Topalov gives up a rook in a sacrifice (often the exchange) and ends up with dominating bishops.
This collection highlights the dynamical nature of the value of the chess pieces. A piece that controls important squares is a valuable piece!