Without a doubt, the hardest reservation to get it the Fantasmic Balcony Dessert reservation. Unless you are staying on propery and have VP do it, you must call exactly one month (eg.june 30 for july 30) and as soon as the phones open at 8am pacific time. Actually, to be sucessful, you should have 2 phones dialing at the same time and then speak really quickly with no chit chat or you might lose out on the tickets esp. if you want an early show.
As for restaurants, none are hard unless, like the pp said, you wait too long. When I call at 60 days out, the CMs even sound surprised-lol. The worst line with no reservations is Rainforest Cafe at dinnertime on a weekend.