I agree with Scrump. To me the word "definitive" implies a version which sets the standard - in the sense of of correctness rather than a standard of excellence.

The word seems to be being misused in this thread and as definitive implies a correct version then there is no such thing for a traditional song as there is no such thing as a correct or wrong version.

Even with pop songs, while the original recording may be considered definitive, in fact I don't think it is as others who may record the song will make different arrangements.

I just checked my dictionary and the first three meanings given for definitive are;

1. serving to decide or settle finally
2. most reliable or authoritative.
3. serving to define or outline.

The other meanings given are specialist ones, though in the spirit of the three above. I don't think definitive is being used in any of these senses in this thread.