Billie Piper "Because We Want To" (1998). Despite reaching number-one on the UK Singles Chart summer of '98 this catchy number was outshined by Britney's ("Baby One More Time") which was released a couple months prior. Bille Piper followed up the hit with 2 more #1s ("Girlfriend" and "Day & Night"). Her debut album, Honey To The B, wasn't the commercial success it should've been.
JoJo "Leave" (2004). I guess JoJo burst onto the pop scene at the wrong time, her debut single "Leave", was a top 40 radio success, but failed to connect to the US Billboard Hot 100, peaking just outside the top 10, at the time you would've only caught rhythmic and urban-orientated songs inside the top 10 mostly. Her self titled debut was amazing, just to note, commercially it did surprisingly well.
Sugababes "Overload" (2000). Back when it was the original three, the Sugababes released their debut, "Overload", back in 2000, although it wasn't until the release of ("Freak Like Me") and the release of their sophomore album, Angels With Dirty Faces, the following year, the Sugababes were brought to the forefront of british pop music, "Overload", still managed to give the, their first top 10, peaking at #6 on the UK Singles Chart.
Christina Aguilera "Genie In A Bottle" (1999). Britney's biggest rival back in 1999, although Christina obviously had the better voice, arguably Britney became more popular, It may sounds really weird to say, but I didn't particularly care for, "Genie In A Bottle", I always preferred ("Baby"), It wasn't until she whored it up in her ("Dirrty") stages, I started to become a fan.
Mandy Moore "I Wanna Be With You" (2000). Technically this wasn't her debut, her real debut was ("Candy") from the previous year from her initial debut album, So Real, but the following year the album was remixed and produced, "I Wanna Be With You", which was the first time I ever heard of her. The song is among my favorite pop songs of all time. It's accompanying album of the same name was also a great pop album.
Jessica Simpson "I Wanna Love You Forever" (2000). Although some may look at Jessica as a joke now, back in her prime I thought she was amazing, her debut single, the vocally powerful, "I Wanna Love You Forever", was her highest charting single to date, peaking at #2 on the Hot 100.