If I only had 1 Day in Orlando, I'd put them in this order:
1. Universal Studios
2. Islands of Adventure
3. Disney Hollywood Studios
4. Magic Kingdom
5. Epcot
6. Animal Kingdom
If you went to, and loved, Disney World as a kid - then for the nostalgia factor, I'd switch #3 and #4.
If you are a foodie and want to indulge, then #1 and #2 become a dead heat and you can switch #5 and #6. Epcot has a tremendous variety of foods available, but it will only encourage you to eat more crappy food. It's like taking a culinary tour of elementary school cafeterias around the world.