Vol 1. - 3 - no contest. The stark black-and-white makes it so iconic for me!
Vol 3. - 4, with its Escher cover, is intriguing and could make people wonder what's inside (which a cover should do!); 5 has flying horses!
Vol 4. - I love to play Where's Wally on #2; or (laughing with it, not at it) #3 has a very nice, simple image - that reveals so much. Aurora's shattered psyche on display with a single head shot.

~ Le Messor
"By the time your face clears up, your brain starts going fuzzy."
~ Anonymous