Tree of Life, with Cinderella Castle a close second.
Magic Kingdom is my favorite of the 4 WDW parks and I love Fantasyland and the Disney Animated Classic films it is based on more than any other land at WDW...
But, I also LOVE animals, and the Tree Of Life - with it's carvings, symbolism, and message - inspires me more than the other icons. Plus, as others have said, it houses an attraction, It's Tough To Be A Bug, which is one of my faves in all of WDW.
I do like to walk through the Castle whenever possible and enjoy the mosaic murals which tell the story of Cinderella, and I also enjoy the nearby Cinderella Fountain and Cinderella Wishing Well, so it is really close between the Tree and the Castle.