But this is one album. Surely to talk about the greatest band we need to look at sales figures of their entire catalog
http://www.therichest.com/entertainment/the-top-10-best-selling-artists-of-all-time/
No - you're confusing 'greatness' with 'popularity' again.
You could say that her 'best' work is greater than their 'best' work. Both of them are beaten by Shania Twain, by the way... (Michael Jackson's Thriller is top of the list)