I'm bumping this up, just because with things like this they're bound to change over time.
I'm going to go with...
Time To Get Alone - The Beach Boys
Most people, unless you're a big fan or just happened to come across somehow, probably don't even recognize this one. It's never played on the radio, was never a single, and is hidden away as a gem on a relatively unknown album. But absolutely amazing singing in this one, and a great harmony.