OMG Rihanna her Greatest Hits would be the best thing ever. Her songs have pretty much all aged well too (poor Gaga). Wouldn't be surprised if it ended up being her highest selling album in the future (which wouldn't be that much of an impressive achievement but still).
Rih's GH will sell max 2m
People are already own these songs anyway and GH albums don't seem to sell much anyway, unless you're an iconic legend like P!nk.