From I can't quit you (babe)": "b-b-b-b-b-b-b-b-babe babe babe babe babe babe baybe I am gonna leave you".
Course, that's not a Plant/Zep original. Babe I'm Gonna Leave You was originally a folk song.
http://en.wikipedia.org/wiki/Babe_I'm_Gonna_Leave_You
and
http://en.wikipedia.org/wiki/Anne_Bredon
It also depends on the type of hit song. Yummy Yummy Yummy was a huge hit, as was Chirpy Chirpy Cheep Cheep.
Personally, I'm with the 'good lyrics' tendency.
Beatles, Beach Boys, Kinks, Smiths (apparently) and many others had huge hits with real lyrics that told a story. There may even be later examples, but I'm an anachronism and therefore wouldn't know.
Stardust lyrics
And now the purple dusk of twilight time
Steals across the meadows of my heart
High up in the sky the little stars climb
Always reminding me that we're apart
You wander down the lane and far away
Leaving me a song that will not die
Love is now the stardust of yesterday
The music of the years gone by
Sometimes I wonder why I spend
The lonely night dreaming of a song
The melody haunts my reverie
And I am once again with you
When our love was new
And each kiss an inspiration
But that was long ago
Now my consolation
Is in the stardust of a song
Beside a garden wall
When stars are bright
You are in my arms
The nightingale tells his fairy tale
A paradise where roses bloom
Though I dream in vain
In my heart it will remain
My stardust melody
The memory of love's refrain