Although Amazon.com sales cannot be extrapolated to represent all sales in general, it can give an idea of which U2 albums are the most popular. The last I saw of the Billboard top 200 Catalog albums, only The Best Of 1980-1990 was on there selling just in the top 100 range.
Anyways here are the current sales ranks from Amazon.com
The Best Of 1980-1990*#129
The Best Of 1990-2000* #199
The Best Of 1980-1990 #360
The Best Of 1990-2000 #690
The Joshua Tree #792
ATYCLB #821
Achtung Baby #1,349
October #1,890
The Unforgettable Fire #1,891
WAR #2,388
Rattle And Hum #3,056
BOY #4,217
Under A Blood Red Sky #5,976
Zooropa #9,181
POP #14,254
Wide Awake In America #23,468
The * refers to the limited edition releases which seem to be selling very well through Amazon. I don't know how October is ranking so high at the moment, but its cool.