I know many of my friends would go for 'You Get What You Give' by the New Radicals, and it's up there, but Gregg Alexander loses marks for writing for Ronan Keating afterwards.
Nah, the greatest one hit wonder was this, no.2 in John Peel's Festive Fifty in 1977, no.1 in the UK singles chart in early 1978 and still sounding great:
Althea and Donna -'Uptown Top Ranking.' mp3
This can probably be found on quite a few reggae compilations and the album is definitely on iTunes. everyone should own at least this song.
There's a wee entry on the fun girls here at Wikipedia