Depends on what you like. Rides? Do Hollywood studios, rockin' roller coaster was my favorite so far, of any roller coaster I've ever ridden, (When they say put your head back, LISTEN!) It also has a fun class there where you learn to draw disney characters. And Toy Story mania is a blast.
Food? Epcot by far. It has a few nice rides, but by far and away the best thing about Epcot is the food. And shopping. If you aren't shy, chat up the CMs, they're all from the country they are working in. Ride Soarin', try Club Cool (stay away from the beverly.) If you're into the characters, this is the park for you too.
But all kidding aside, Disney is Magic Kingdom. That 'awww' feeling of your favorite rides as a kid, the castle? All Magic Kingdom. Personally, I would never go to Disney without stopping at MK. It doesn't feel like a vacation until I put my eyes on that castle. And then go ride It's a Small World (where I promptly bawl. Not gunna lie. Happens every time.)
The park I personally am not a fan of is Animal Kingdom. I feel like there are much better zoos out there, and there is only two saving graces for the entire park. The Yeti, he's my bro, and the Kilimanjaro Safari. I will say that the coolest stories I come back from happen in AK though, for example, when I was taking a few toddlers on the KS, I actually had to pull one of their hands back from touching a rhino, they were that close. That doesn't happen often in a lifetime outside of Africa, but it happened in Disney! If you like looking at architecture, definitely this is the park for you. My absolute favorite architecture is here. If you have the time go through the Kali River Rapids wait line, it's gorgeous. (Just don't ride the ride if it's cold, chances are you'll be soaked.)