Which book of Scripture is most esteemed by the LDS church? I understand that the LDS church regards the Bible, the Book of Mormon, the Pearl of Great Price, and the Doctrine and Covenants all as Scripture.

In practice, which of these works is most highly esteemed as being applicable and helpful to an individual's life?  Is one generally preferred over another?
Because Mormons believe in an open canon of scripture (i.e., that God continues to provide scripture through His living prophet), the words of the living prophet (or president of the church) are esteemed more valuable than ancient records. Every six months the leaders of the church formally address the church and their words in these general conferences are considered modern scripture. (see http://gc.lds.org/)

It's important to emphasize that this doesn't devalue the Bible, Book of Mormon, Doctrine and Covenants, or Pearl of Great Price. Each is singly taught as the official Sunday School curriculum for a year on a four-year rotation (actually Old Testament; New Testament; Book of Mormon; Church History, incl. Doctrine and Covenants and the Pearl of Great Price; see http://www.lds.org/manual/sunday-school?lang=eng). The words of the living prophet are just considered more timely.

The path to salvation will always be the same (the gospel of Jesus Christ), but present-day temptations like debt and drug use aren't explicitly warned about by Abraham or Noah or Moses because they weren't prevalent during their time. Because they are prevalent today, God speaks His warnings the same way He spoke His warnings in times past, through a living prophet who can and does explicitly condemn things like drug use and excessive consumer debt—modern temptations that can make it difficult to live the teachings of Jesus Christ.

See also: http://www.lds.org/liahona/1981/06/fourteen-fundamentals-in-following-the-prophet?lang=eng