though I don';t know her songs well enough to pick one.

It's not that her stuff was unique, but it was unique in that it *crossed over into the pop market* in a way that no other country star had done in at least 15, maybe 20 years... no matter how big Garth got, his popularity remained firmly stuck in the country market. Shania and her abs busted right on out and into EVERYONE's living room (and dentist's office).

If I think of 1999-2000, the pop music that springs to mind:
Britney
pre-Xtina
Kid Rock
Eminem
NKO... I mean, Backstreet & n*Sync
Rickie Martin
J. Lo
Mambo # whatever (which ain't a mambo but could still be considered as a separate entry for purposes of this post)
maybe Janet

The last time I can recall that a country song/artist had gone mainstream was Islands In the Stream (1983), and Elvira (giddyup bawoombop bawoombop, #5 pop hit in 1981).