Tehillim, primarily written by Dovid Hamelech - one of, if not, the best kings in Jewish History. He was considered by G-d as a servant and is used by G-d as the epitome of a king following in His ways.
Tehillim wasn't just written as poetry and song, but had Divine intention. Some of them are used as the accompanying song to Korbanot in the Bet Hamidkash. Others speak about building a relationship with HaShem and coming closer to His service. It has a topic about all things, and is usually the prayer tool of choice when an emotionally-charged situation requires Divine intervention or for praying for the wellbeing of others.