Oh my, that definitely is a difficult question.
I absolutely cannot pick just one, but some of my favorites are:
The High Priestess~ she looks so elegant in her sheer, sparkling gown floating carelessly above the water with a wonderful celestial background.
Six of Cups~ The children playing among the cups filled with flowers, the animals, all the color, this card is just beautiful.
Seven of Cups~ all the cups filled with rainbows and doves, etc., its a breathtaking card.
I absolutely love this deck, it's my first, the one I've been eyeing for quite a few months, and I must say it is MUCH more beautiful in person!