Cinderella's Castle for me (would have been SSE, but that view fell victim to the wand). The Tree of Life is really nice too, but nothing says Disney to me like Cinderella's Castle.
As for the Sorceror's Hat, I hate where it stands (blocking the theater) and what it stands for (Disney overzealously pushing merchandise), but I must say that the Earful Tower was just too bland for me, up in the air on its legs like that. Also, the Earful Tower just seemed too out of the way, not right down the park's main avenue, like all park icons are. I hope that oce the issues are resolved with the theater, that it would become the icon of MGM and the hat will disappear or at least move and take on a smaller role, but I don't see that happening. To me though, the theater always was and always will be MGM's icon, even if it isn't featured on magnets and photo albums with the castle, SSE, and the Tree of Life.