I would do MGM because they have the only two true thrill rides in Tower of Terror and Rock n Rollercoaster. Star Tours is also in MGM which is a thrill simulator and quite good.

Shame to do only one park as you would miss the other ones that I think are pretty good thrill rides, namely :

Splash Mountain @ Magic Kingdom
Mission Space @ Epcot
Dinosaur @ Animal Kingdom

The other rides are good but, personally for me, didn't get me going as the ones I've listed.

Brizzle.