Since it will be 2 adults, I'd opt for a park where you could ring in the New Years with a bit of bubbly! That rules out the Magic Kingdom, which will be VERY crowded anyway. I think I'd also pass on Disney's Hollywood Studios because of the smaller size. The Animal Kingdom will be closed.
So that makes Epcot my first choice since you can definitely celebrate with many choices of beverages and wonderful food if you'd like: but what makes it very nice is that even when crowded, the park can hold crowds much better than the smaller parks. Also, you will both be able to take the time to explore the countries on your own terms that you may not do when you are with your children. During the evening, a special version of Illuminations will be shown twice: 7:40 and 11:50. I've even heard about the countries in the World Showcase ringing in their own New Year's celebration when their country's time zone celebrates 2009.
You'll be bringing in 2009 in one of my favorite places, WDW. Wherever you end up, it will certainly be a magical evening!