I find myself agreeing with Clinton for once.

You think some version is definitive, then you hear someone else sing it, and it defines it differently.

Fairy Tale of New York - how could the Pogues/Kirsty McColl version be matched? Then I heard Christy Moore's, and it did match it.   

I suppose it depends how you define definitive - if it means setting a standard for how good it can be, that's fine. But if it means that the song has been nailed so noone else should feel free to sing it, that means it's killed the song, and I don't go for that

Judy Garland's Over the Rainbow is definitive in the first sense. But I once heard Lonnie Donegan sing it, and it was quite different, and didn't suffer by comparison.