Definately Madonna. I am a huge Madge fan. I've got her 2 disc greatest hits 'Celebration', plus 'The Immaculate Collection' and 'GHV2'. I also have 'Celebration' on DVD, plus three live shows on DVD: 'Ciao, Italia' from 1988, plus 'Confessions Tour' from 2006 and 'Sticky and Sweet Tour' from 2009.
Also, she's been around in the music business since 1983 and has over 50 hits to her name.
She's also been releasing singles a lot longer than the rest of the singers on this list.