I prefer the retro posters like People Mover and Matterhorn. Those two in particular are probably my favorite. I've been tinkering with a House of the Future poster that is more in line with that style than the one they created for Disneyland's 50th. Interestingly, the poster they came out with for the 50th (and subsequently my version) use the Matterhorn poster as a template for the mountain in the background!
Alice in Wonderland would probably be my #1, with Haunted Mansion following it up. I'm also quite fond of the DCA revamp posters they did. Just think the art style is nice, modern yet with a classic touch.