It all depends, I think that the Haunted Mansion is my favorite, but it could be nostalgic! All of our memories are attached to emotional memories too! I love the Magic Kingdom.
For Epcot, I haven't seen Soaring yet or Mission Space for that matter, but my favorite isn't an attraction, it is walking around the countries. I may change my mind once I try these rides, but I doubt it!
Good luck on narrowing down your park agenda! Me, I couldn't do it. Enjoy!