I was thinking about how certain voices just harmonize so perfectly together. Each individual voice may not be fantastic on its own, but accompanied by just the right singer, they create pure sonic heaven. The flip side is that two singers may have incredible voices on their own, but together they sound like crap.
I would have to say my favorite harmonizing duo is Cindy Wilson and Kate Pierson of The B-52's. From bombastically boppy to ethereal, their voices fit together perfectly.
Other duos that come to mind: Lennon and McCartney, and ABBA's Anni-Frid Lyngstad and Agnetha Fältskog could be argued as the best of all if they hadn't been so highly overdubbed. There's probably some singing duo that blows everyone away that I've forgot (no, it's not Hall & Oates), so I welcome any suggestions.