Racking up eight No. 1 albums and taking 14 singles to the top of the charts, Westlife conquered the world of British pop in the '00s. Formed in 1998, they were mentored by two industry legends: producer Simon Cowell and manager Lou Walsh. Initially competing with teen pop sensations like the Backstreet Boys and *NSYNC, Westlife kicked off their career with a run of infectious singles driven by slick R&B hooks. The group really hit their stride with a string of moody and moving ballads that won them a new adult audience, but the pressures of stardom forced the band to call it quits in 2012.