Without delving into research I venture 1 guess anyway. I'd say ABBA is one.Album sales by a group.
I did the research and according to my sources, the most selling artists worldwide are
ABBA 370 million
AC/DC 200 million
Alla Purgacheva 250 million (Russian pop)
A.R. Rahman 200 million (film score/world beat)
Aerosmith 150 million