My top 5 appear in no real order. What will surprise most is the absolute lack of signature dining apart from California Grille. Sometimes, dining in Disney is about the experience that accompanies your meal, and the appreciation of a nostalgic or simple dish that's just perfectly done.
1. Teppan Edo (EPCOT) - it's a Japanese steak house, which most cities have, but it's recently been remodeled and is absolutely BEAUTIFUL inside now. It looks like a chic Japanese night club, but without the pretentious atmosphere. The chefs rarely falter (unlike aforementioned city steak houses), cook fabulous food, and are usually really great with kids. Their sushi is fantastic, too!
2. California Grille (Contemporary Resort) - sure, it's okay for kids, but with the gorgeous views, and the ability to leave your table and watch the fireworks on the private viewing terrace, it's one of the most romantic locations in WDW. The food is phenomenal, and they know how to mix a really great Cosmo. The steak is flavorful, well-accompanied, and can be cut WITH A BUTTER KNIFE.
3. The Margarita Hut (Downtown Disney and EPCOT) - it serves "liquid meals." These are some of the best frozen margaritas I've had. If you're feeling adventurous, go for the Fiesta Margarita, drink it quickly (watch out for brain freezes!!!), then hop on the river ride in the Mexico and feel the buzz of good tequila wash over you.
4. Wolfgang Puck Express (Downtown Disney) - if you're on a dining plan, this gives you major bang for your buck, as it's a counter service meal that would normally cost you about $30 with everything included. Based on Puck's popular Californian/fresh cuisine stylings, you HAVE to try out the delicious frozen yogurt, and the innovative pizzas are excellent. Sandwiches border on OK, but everything else is great!
5. Casey's (Magic Kingdom) - these are a stolid family tradition. Of course, you always should beware the "Lucky Dog", which can cause you to become all-too-well acquainted with the Magic Kingdom's restrooms, but the experience of eating just a perfect hot dog outside with your friends and family is great. Casey's is all about nostalgia and a personalized meal - chili? cheese? relish? saur kraut? Put whatever you want on that dog, and then wash it down with an enormous soda and a side of Cracker Jacks.
Bonus: I haven't tried it yet, but The Wave at the Contemporary Resort has been receiving all types of praise... it's organic, much in the vein of Cali. Grille, but is only 1 Table Service instead of 2. I'm going in September and can't wait to sample!