But the best flavor text ever is still Martyrs' Tomb.
I don't know, I find the Wall of Vapor quote inspirational, as well:
Walls of a castle are made out of stone,
Walls of a house out of bricks or of wood.
My walls are made out of magic alone,
Stronger than any that ever have stood.
From Shattered Perception (Discard all the cards in your hand, then draw that many cards.):
"You must shatter the fetters of the past. Only then can you truly act."
I think this one takes the cake, in terms of rationality.
Here's the new thread for posting quotes, with the usual rules: