Geez, this is going to sound awful but I would answer with “most of them.” I enjoy the parks as a whole, but there are very few attractions I consider must-do for every trip. I’m going to do this exercise in reverse and list the attractions I do try to do every time and would be disappointed to miss, as there are only a handful to name.
MK — CoP, I like this one for the nostalgia factor from trips of my youth.
EPCOT — Nothing is coming to mind, though I probably like Living with the Land the best.
DHS — I could pass on this whole park.
DAK — The Safari, Affection Section, and the animal viewing trails. I’ve only done FoP once due to FP availability but would definitely like to ride it again.