Three songs come instantly to mind for me;
1) Sympathy For The Devil (Stones). Female backup vocals credited to both Marianne Faithful and Anita Pallenberg. I could be wrong but I hear a single, primary female vocalist throughout most of the song. Recent accounts credit that outstanding performance to Pallenberg.
2) The Great Gig In The Sky (Pink Floyd, DSOTM). Clare Torry's powerful vocals absolutely drive this song. The track is short on spoken lyrics to begin with and Torry's vocals add no words. A backup vocalist for the album, but a primary vocalist here.
3) Paradise By The Dashboard Light. (Meatloaf). Okay, maybe more of a duet. Still, it would be unfair not to credit Ellen Foley for one of the more impassioned vocal performances when not the premier artist. I think she actually out-sings Meatloaf on the song (not an easy feat). Album credits call Foley the "featured female vocal" on that particular track. Given @fatcat 's mention of Todd Rundgren above, I'd be remiss not to say that he is also included in backing vocals on the song as well as the producer of the album.
This thread already takes long enough to load, so for courtesy I've forgone imbedding links assuming anyone interested in hearing the tracks is capable of Googling them.