For the longest time, I would have told you that Serra Angel was my favorite for the reasons of beauty, simplicity and nostaglia. But right now, my favorite angel is probably Pristine Angel, for what she's done for me in actual game play. Serra's efficient, but Prissy's a house for just one more mana. In MTGO limited, she's one of those "right click -> concede" cards. And the art's not too shabby, either... especially if you can check out the full length version and not just the segment that appears on the card.