Many people equate Walt Disney World with the Magic Kingdom, but there are actually four parks that require separate admission (or a Park Hopper pass). While you can enjoy your day at any of the four parks, here's some advice if you have to choose just one:
The Magic Kingdom is ideal for families: This park was the first, and it's still the most popular. It's a classic that recently got a major revamping with the additional of New Fantasyland. If you have young kids, the Magic Kingdom is an especially great park to visit if you can only choose one. The rides are family-friendly, there are plenty of character greeting opportunities, and the Wishes fireworks are always a great way to cap the day.
Epcot has an older vibe, particularly in World Showcase. It's fun for all ages, but if you've got a party of adults, it's a great place to spend your time if you have to pick just one park. You'll enjoy Future World, too, which has some rides with an educational twist, like Spaceship Earth and Ellen's Energy Adventure, and some that are pure fun, like the Nemo ride and Soarin'. Then enjoy the movies and entertainment in a walk around the "world" and even indulge in an adult beverage from among the intriguing choices.
Disney's Hollywood Studios has a good blend of rides and spectator events, with a helping of Pixar thrown in. For example, the Tower of Terror, Star Tours, and Rock 'N' Roller Coaster pack some thrills, but the Indiana Jones and automotive stunt shows, Beauty and the Beast, the Little Mermaid, the American Idol Experience, and Muppet Vision 3D are all show examples.
The name says it all for Disney's Animal Kingdom. It has rides like Dinosaur and Expedition Everest, but its main draw is the animal residents. This is a great park for a laid back day of just strolling around, soaking up the atmosphere, and looking at all the animal habitats.