1) Cinderella's Royal Table
2) Chef's Table at Victoria and Albert's
3) 'Ohana dinner
4) Chef Mickey's breakfast
And now, pre-park opening Akershus and Garden Grill have been tough to get, as people try to beat the rush to Soarin' and Frozen Ever After.
It can also be tough to get Rose and Crown around the time Illuminations starts and Beaches and Cream for lunch or dinner, but they can be had. Not sure how hard it is to get Le Cellier since it became 2 credits on the dining plan.
I find these are generally the toughest.