A team player who's always game to mix things up, Usher uses his pair-ups with other artists to flex his creative muscles and add new spice to his R&B brew. This playlist provides an in-depth tour of the singer's cooperative efforts: Nicki Minaj and Rick Ross show up on slinky bangers, P. Diddy flirts with low-key rhymes and Alicia Keys and Mariah gild deeply romantic slow jams.