Seal never really established a single definitive sound. The thread that ties his almost twenty-year career together is surely his voice. Breathy but not husky, sensitive but not sentimental, it often feels like a masculine equivalent to Sade’s mysterious coo. The various soundscapes that appear on this greatest hits collection are mostly the work of songwriter and producer Trevor Horn, whose extended collaboration with Seal is unique in a career defined by one-off projects for everyone from Yes to Pet Shop Boys to Belle & Sebastian. Horn is responsible for “Crazy” and “Kiss From a Rose,” Seal’s two most memorable songs — the first a hypnotic house-inflected tune in which Seal’s voice blends completely within the contours of the music, while the latter is a stately and ornately orchestrated ballad that won Horn and Seal a Grammy award in 1996. But the most welcome surprise of Best, 1991-2004 is its covers. Seal turns Steve Miller’s “Fly Like An Eagle” into a sequel to “Crazy,” while “Walk On By” remains moody and bluesy beneath a techno beat. Some Best Ofs suffer from jarring diversity, but Best, 1991-2004 makes an asset of its varied terrain. Its multi-layered playlist goes to show that all Seal has to do to make a song his is sing on it.