Intuition & instinct tell me that the ones that haunt your dreams are sure to be the most ancient. I've been haunted by Child #19 for the past 30 years and I'd say it was pretty ancient in terms of language, melody, structure, and subject matter of course - in terms of mythic analogue you don't get much older really. Of course if it turns out to be rather less so I won't mind too much. Other's haunt similarly - #32 is effective on various levels (though English versions invariably miss the essential humour of the thing) whilst I believe #102 to be one of the finest things ever written. In the absense of a melody, I set it to the tune of Adam de la Halle's Bergeronnette Douce Baisselete back in 1990 & we still sing it today - we've a version featured on John Barleycorn Reborn as well as an ambient remix featured on the forthcoming John Barleycorn Rebirth CD. With our ballads session coming up at Fylde festival next week, I've just uploaded the basic live take onto our Soundcloud page which captures the inner essence of the thing (though we no longer use the accordion drone):

Rapunzel & Sedayne : Child #102