FWIW my reasonably confident* guess at best-selling 21st century artists using a literal definition of sales (no distinction between albums/singles or physical/digital, streaming doesn't count) is 1. Eminem 2. Rihanna 3. Taylor, with RiRi having the advantage of her 2007-16 Imperial phase corresponding exactly with the iTunes era. Of course this mostly shows that pure sales isn't the best way to make comparisons even pre-streaming: for one thing live shows should count (it's the biggest revenue stream for Taylor among many others these days) though pure revenue also shouldn't be used too literally or you get things like Depeche Mode (!) being one of the biggest artists of 2023.
*We have pretty good approximations for US and Western European sales these days; Asia remains mysterious outside of Japan and maybe Korea
I like the turning-oranges-into-apples formula that ChartMasters uses in theory but don't trust their methodology at all, and their rankings look positively screwy to me at first blush https://chartmasters.org/rankings/
What is helpful about their charts, though, is they break out different metrics, where you can see that Rihanna's dominance comes almost 100% from digital downloads, while Taylor Swift has twice the number of albums sold and twice the streaming numbers (if they've calculated streams fairly, but this squares with how I eyeball it).
(I did go back and tweak that footnote though to say that Rihanna is probably below Taylor and MAYBE below Drake, though that's debatable. For one thing I really don't think you can discount the years 2016-2023 and not try to account for streaming!)
FWIW my reasonably confident* guess at best-selling 21st century artists using a literal definition of sales (no distinction between albums/singles or physical/digital, streaming doesn't count) is 1. Eminem 2. Rihanna 3. Taylor, with RiRi having the advantage of her 2007-16 Imperial phase corresponding exactly with the iTunes era. Of course this mostly shows that pure sales isn't the best way to make comparisons even pre-streaming: for one thing live shows should count (it's the biggest revenue stream for Taylor among many others these days) though pure revenue also shouldn't be used too literally or you get things like Depeche Mode (!) being one of the biggest artists of 2023.
*We have pretty good approximations for US and Western European sales these days; Asia remains mysterious outside of Japan and maybe Korea
I'm a little skeptical of the most liberal estimates of Rihanna's sales during the digital download explosion because she never shows up in the other metrics that people were writing thinkpieces about for Taylor (including album sales in the first half of her career and streaming in the second half). Then again, by some estimates of album sales alone, Adele is the powerhouse, even though her net worth is puny next to Taylor or Beyoncé -- not to mention Rihanna, who seems to be *much richer than both of them* but not because of her music.
I like the turning-oranges-into-apples formula that ChartMasters uses in theory but don't trust their methodology at all, and their rankings look positively screwy to me at first blush https://chartmasters.org/rankings/
What is helpful about their charts, though, is they break out different metrics, where you can see that Rihanna's dominance comes almost 100% from digital downloads, while Taylor Swift has twice the number of albums sold and twice the streaming numbers (if they've calculated streams fairly, but this squares with how I eyeball it).
I think it's generally safe to conceptualize a true superstar class for the 21st century that includes Eminem, Taylor Swift, Rihanna, and Drake in some order, and maybe Beyoncé, especially if you include Destiny's Child (though I think people overestimate how comparable Beyoncé's footprint is to the others). Hard to say with acts that had mega sales between 1998 and 2000 like Britney and BSBs, and I hope that my total loathing of Adele doesn't make me discount her sales.....
(I did go back and tweak that footnote though to say that Rihanna is probably below Taylor and MAYBE below Drake, though that's debatable. For one thing I really don't think you can discount the years 2016-2023 and not try to account for streaming!)