Hmm. tough call.
Epcot is a must, so that's out.
It's amost a toss up on the other 3.
MK - OBviously, I love... well, loved, the MK, but it has suffered the most from the blatant kiddification/characterification, that is almost saddens me to go. Since I don't have kids, I feel like it's not for me. On the other hand, the things that are left put me in that kid mode again. So, it's a very schizophrenic park experience for me. :lol:
MGM - As a park, it is SORELY lack IMHO. I don't like Fantasmic, the atmosphere is blah and disjointed at best, and it is lacking on attractions. But, it has three of my absolute top WDW attractions (TOT, RnRC, Muppets).
DAK - This park is really starting to grow. It is more balanced than MGM, but it just doesn't have that much. KS, Dinosaur, EE, and ITTBAB are all great attractions, but they don't fill a day, and they do have somewhat limited re-ride. I really feel if they got a strong additional section like a Beastly Kingdomme or something like that with 2 or three signature, all day attractions, it would easily beat MGM if not MGM and MK.
So, all in all, MGM would drop off, but that's what park hoppers are for, right!