It's only underrated by delusional and pressed people.
It's not at all forgotten, it's a (true) classic that already stood the test of time.
I was at a party last saturday night and I was dressed up in the school-girl outfit like Britney in the video and EVERYBODY, I mean EVERYBODY knew what I was :
- "You dressed up as Britney Spears, aren't you?"
- "YESSS mate !"
Another one:
- "Hey you're Britney right ? We're gonna put "...Baby One More Time" and you're gonna sing it !"
- "Hum yeah sure"
It's a fact that "...Baby One More Time" and all that's around is an important part of pop culture.