Why is the light-squared bishop considered White's best bishop? The same question applies to Black where the dark-squared bishop is considered Black's best bishop.

This was from the book "Logical Chess Move by Move" by Irving Chernev where he quotes Siegbert Tarrasch as saying the King's Bishop is the greatest attacking piece, so I guess it does pertain to mainly one player with one school of thought.  I found it interesting how Tarrasch at times would not even move his King's Bishop during a whole game.
The black king, will occupy e8 and g8 (before and after castling, respectively), both of which are white squares.  Also, the weakest point on the black kingside is f7, again a white square.  Because of this, if you are trying to attack the king, the white bishop can often be more valuable.  

Perhaps someone else can add some more deeper reasons, but this has been my experience.

Added: Also worth noting, attacks on h7 can often threaten mate, another white square.  Furthermore, just having the bishop on the diagonal attacking f7 would pin the pawn to a castled king.