I think it's Sometimes, Criminal and If U Seek Amy.
Sometimes is her 9th most viewed solo song in Youtube, having 215m views, towering above a lot of her other songs. Not that it's unexpected that this song has longevity, but more on how much longevity and popularity it has compared to her other songs.
Criminal has 117m spotify streams, the most among the Femme Fatale songs.
If U seek Amy has 90m streams on Spotify.
Both Criminal and If U Seek Amy are in the top 10 most popular tracks in her Spotify page (as of date of this writing)