Zac Efron is, first and foremost, a film star rather than a musician, which makes his appearance here a quirk of how soundtracks accumulate streams. His singing career exists almost entirely through two movie franchises: High School Musical, which made him famous, and The Greatest Showman, which gave him his biggest song.
That song is "Rewrite the Stars", his swooping 2017 duet with Zendaya from The Greatest Showman soundtrack. Carried by the film's enormous and enduring popularity, it became a streaming hit far larger than his earlier High School Musical numbers. It now sits near 1.2 billion plays.
His other charting tracks, mostly from High School Musical, trail well behind, with "Gotta Go My Own Way" his next biggest at around 241 million. That puts the ratio at about 5.17, just over our 5.0 line.
So by our strict, numbers-only measure, Zac Efron registers as a certified one-hit wonder, with an obvious caveat: he is an actor whose recorded output is a by-product of his film roles. One blockbuster-soundtrack duet has simply outstreamed the rest, which says more about the reach of The Greatest Showman than about any music career.