Llewellyn's Priestess is Ceridwen, goddess of the witches. She has the cauldron of inspiration and seems to be part of the landscape. In fact, she seems to stand for nature itself. I love all the flowing water in this card.
Marchetti's Guilded Priestess has already been posted here, but I like to compare her to the others. She is a very dreamlike figure, floating in air in profile, rather than the usual frontal hieratic position. Her water world is unveiled and the moon symbol very prominent, reminding me of the Moon card, the subconscious. This one is probably my favorite.
The Universal Priestess is very outlandish, floating over an alien landscape on her throne. There are a couple of masks at the front that have me quite perplexed. Once again, this is the one I find it harder to relate to.