what is the best chess piece?
Some old chess rule books value the King as "infinite" in value since the game ends with checkmate (equal to losing the King). As for the other pieces, it is all relative to the position. Give Petrosian Knights in a closed position and they are worth more than Rooks; give Tal a Queen and it is said that his Queen is worth 12 points (rather than the traditional 9) and then there are of course obvious common examples like fianchetto Bishops worth more if the long diagonal is under its control.