Oooh, this is a tough one...
1) I know my first would be my Filigree Angels. It was the very first foil card I ever pulled in a booster (I started playing when the Shards of Alara block was current), the artwork is GORGEOUS, and it is the reason that I play Blue/White.
2) Venser, the Sojourner. I just recently acquired him (Best Friend bought him for me), and I am so excited about my deck featuring him. The artwork makes him look like the Tenth Doctor, and his abilities lend for some very interesting variations on UW control decks.
3) Chandra Ablaze. I'm a sucker for fire images, and I have always said that if I could have a superpower/magical abilities it would be pyrokinesis/pyromancy. Chandra embodies that and the artwork on this rendition of her is stunning. I like Chandra Nalaar's ultimate better, but strategic use of Chandra Ablaze could end a game really quickly.
-RLB