My own favourite is "Vivaldi's 'Rain'," with "Sigma" a close second. I'm thinking the criteria could be a mixture of things -- the setting, the wardrobe, the song itself, and the way in which Chloe sings it.
For example, I love the setting of "Sigma," I adore the outfit Chloe wears in "Vivaldi's 'Rain," I enjoy "Panis Angelicus" as a song (but I prefer the Slane Castle version as a performance), and I admire the way in which Chloe sings "Brahms' Lullaby."
Whatever makes a video a great overall experience to you.
Kimberly has kindly listed YouTube links for all of the videos from the DVD here:
http://www.celticwomanforum.com/index.php?topic=1443.0in case anyone wants to watch them again, to decide on a favourite.