I think the combination of TOT and RNR at MGM has really helped to increase attendance there. I remember going there because it was always pretty slow, but that doesn't seem to be the case anymore. I guess it's a good thing though!
It's got to be ToT and RnR that pulls them in for MGM.
For AK maybe ITTBaB and KRR.
EPCOT is Test Track, though i personally find it a little disappointing.
MK, it has to be the Mountains, Space and Splash.