Some absolutely banging duets on here, but I'm afraid you're all wrong
Given the history how it came about, and also their awesome live performance, the best duet has to be neil diamond and barbra Streisand "you don't bring me flowers"
Long story short, Diamond wrote and recorded it, Streisand covered it, a US DJ came up with the idea of splicing together tapes of the two into a version of them singing against each other, it went viral on the radio long before viral was a thing. The two of them eventually went into studios a couple of years later (I think one in LA one in New york) and recorded their tracks which was then mixed and released as a proper duet.
When they were asked to do the 1980 grammys, they had a practice the night before on a crackling phone, and then went on the next evening to performing it together for the first time and gave the most wonderful performance...and never sung it together again.
Actually, that's not quite right they both appeared at a democratic fundraiser and sung it in the early 2000s unfortunately only a really crummy audio version of it exists.
I'm no Streisand fan but the live recording of her and Kris Khristofferson doing evergreen is also something to behold.
It appears she was a hell of a singer when she was younger, and was able to have great chemistry with partners.