I like Jerry's question, and especially McGrath's erudite comments.
But even "original" has its problems. John Denver's first original recording of "Rocky Mountain High" is noticeably slower and more melancholy than a recording I have closer to his death, which is zippier and more upbeat. Conversely, I have an early recording of "I Guess He'd Rather Be In Colorado" which is flat and unremarkable. Yet, many years later he recorded a version that is full of emotion and much more expressive. Which would be the "original" original, since he "originated" both songs?
For me, (for better or worse) definitive also happens to be "that which I heard first or the most frequently." Ergo, PP&M singing "Blowin in the Wind" is definitive, as is the Kingston Trio singing "Tom Dooley" and "The Sloop John B." I've heard others, some predating said covers, and somehow they just don't sound "right" to my ears.
Still, Bing's "White Christmas" cannot be matched. Nor can Louis Armstrong's "What a Wonderful World". And I also have to mention "The Little Drummer Boy" as performed by the Vienna Boys Choir. Something about boys voices telling the story about a little boy just fits the song more naturally than Celine Dion (or any redneck cowboy, fer cryin' in the grog).