Epcot and MK are a close, then there's a drop off to DHS (still want to type MGM), and another drop to AK. I couldn't do a WDW trip and skip either MK or Epcot, but could and have skipped the others.
My order is:
1) MK
2) Epcot
3/4) DHS & AK - depends on my mood, season we're traveling, how long of a trip and who's with me. We like specific things about each of these parks but not all of it. I do love the afternoon parade at AK though!
We will regularly go 2 or 3 trips and not bother with DHS. AK is about an every-other trip visit and we never bother with the waterparks, since we live in WI.