Poker Face on both counts. Umbrella aged terribly and is overshadowed by Rihanna's more recent songs like We Found Love. Poker Face turned out to be truly timeless, and is still one of Gaga's most fondly remembered hits.
That song didn't sell 15m maybe if you add streaming and massively inflate those numbers because otherwise it isn't even past 13m.
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OT: Umbrella was bigger and is still bigger nowadays.
14 or 14,5M is the official number (no inflation, no streaming) but it's an old figure, since it's still getting catalog sale and views '(more than Umbrella since PF is way more remembered) we can guess it sold around 15M WW
maybe if you add streaming and massively inflate those numbers because otherwise it isn't even past 13m.