Oh, boy, you're gonna stir up some wide opinions with this topic! lol Seriously, you will likely get a ton of "must eat" suggestions, so I'd follow up on the ones that appeal to you the most by looking at menus. That being said, here is my short list:
ARTIST POINT: Far and away my favorite restaurant. It's tucked away at the Wilderness Lodge which may explain why it never seems to be crowded, yet it's a 2-credit signature restaurant. I've eaten there 6-7 times over the past few years and have never had a bad meal there nor poor service. The ambience is superb, especially if you ask them to seat you by a window (you might have to wait a few minutes). After the meal, stroll around the gorgeous lobby (the GCH comes to mind), watch the geyser blow, sit on the beach, and watch the Electric Water Parade. It's a grand experience.
CALIFORNIA GRILL: Rather hard to beat the view (again, you can ask to be seated by a window, but it might take awhile to get one of those seats), and the food is innovative and tasty. I've found the service to be good. It's not as relaxed an atmosphere as AP and generally a lot more crowded, but I do enjoy it immensely. Also a 2-credit signature restaurant.
JIKO: Located at the Animal Kingdom Lodge, the flatbreads are tremendous. . .but very filling! The lobby at the AKL (the main, older lodge that is called Jambo House) is nearly as awe-inducing as WL. You can't stroll there quite as much as at WL, nor are there any views to speak of. But, the restaurant is superb with equally gracious service.
After those three, there are many other fine choices, but I'm sure others will post their favorites in due time. Have fun deciding!