There are a couple of songs off of GnR's "Spaghetti Incident" where the backup overshadows Axl. Particularly "Ain't it fun" (Michael Monroe from Hanoi Rocks).
Maybe Mick on "You're so vain"?
"take me home tonight" is on the short list of songs where the backup singer did a better job than the lead. I'd add Paradise by the Dashboard Lights to that list. Others? Both mick and his backup are great on Gimme Shelter, so that's not one. And I don't mean any fleetwood mac song that has Christie McVie singing backup. That's too easy.
