My wife and I were listening to a R & B Christmas CD I put together a couple of years ago. When The Christmas Song came on by Nat King Cole, we both thoroughly enjoyed hearing it for the millionth time. The song was written by Mel Torme, and he does a wonderful job on it. I offered to play it for my wife, but she knew that no matter how good it might be, nothing could ever top Nat King Cole's version.
Some recordings are like that: someone nails the song so completely that it seems like no one else could ever equal, let alone top it.
Of course, all of this is extremely subjective. That said, there are several recordings that come to mind that, for me, are never likely to be equalled. In that category, I'd put:

   Duncan and Brady, by Dave Van Ronk
   Silent Night, by the Temptations (on the same Christmas CD)
   Fannin' Street, by Leadbelly

I picked these songs, because I've heard other people try to do them, and even though the version might have been fine, it couldn't touch the definitive version (my definitive version.)

Jerry