MK has the most rides and the most selection. (And is the most magical.)
I second this! The MK has the largest number of attractions, and the smallest footprint. Highlights (which can easily be done in 6 hours, with a smart touring plan and use of FP+) include Pirates of the Caribbean, Splash Mountain, Big Thunder Mountain Railroad, the Haunted Mansion, Peter Pan's Flight, it's a small world, Seven Dwarves Mine Train, Dumbo, Space Mountain and Buzz Lightyear's Space Ranger Spin. On a second go-round, I'd select a few out of: PeopleMover, Carousel of Progress, the Monsters, Inc. Laugh Floor, Winnie the Pooh, Journey of the Little Mermaid, Mickey's Philharmagic, the Muppets "Great Moments" show, Country Bear Jamboree, the Jungle Cruise, Enchanted Tiki Room and the Swiss Family Robinson Treehouse.
It might depend on your individual interests, though. The Animal Kingdom (large footprint, lots of walking but some unique and beautiful attractions) would be my second choice.
Obviously, whichever one you choose, you'll have to decide ahead of time what the highlights of the park are for you, and confine yourself to those. A few dollars for a touringplans.com subscription, so you could work out a personalized touring plan that maximizes every minute of those six hours, would be well spent.
