I have a couple of favorites. The first one is in a bush in the miniature train garden in EPCOT. One of the bushes has been trimmed into the shape of a mickey and is very hard to see. The other is in the Haunted Mansion. It's in the grave yard after you pass the four singing guys. To the right of the opera singing lady is a grim reaper. He is holding up his hand and hanging from his hand is a classic hidden mickey.
We love looking for hidden mickeys. My four-year-old is an expert at finding them everywhere...grocery stores, puddles of oil in our garage. We found one on a tree in Gatlinburg, Tennessee.