After a quick check on wikipedia:
Appetite For Destruction: 18 million copies sold in the US
Fallen: 7 million copies sold in the US
Both albums have sold great but AOD has Fallen beat by a good margin. I knew AOD had sold a ton but even i'm taken back by how many copies it's actually moved
It's in the same company as Led Zeppelin 4, Dark Side Of The Moon and Back In Black with sales. Which is very impressive.