This is a tough question with certainly no one correct answer. For us, we love the opportunity to try new things that dining at Epcot offers, so we've always appreciated Restaurant Marrakesh since we don't have close access to Moroccan food; and how often do you get a chance to belly dance during dinner?
Similarly, we have fun at Biergarten's Oktoberfest atmosphere and a chance to "dance" and not care how we look ! But my oldest who is taking German in school doesn't like going there anymore because I want him to speak German the entire time.
Some other favorites are Le Cellier, Tutto Italia, and Rose & Crown (this could be timed for a late dinner to enjoy dinner and Illuminations at the same time), but I think the one I'd recommend the most is the Coral Reef. I think it is a beautifully decorated restaurant that makes us almost feel as if we are part of the ocean with its deep blues and subtle lighting, but what makes it even more amazing is that one entire wall is an aquarium. Watching the sharks, turtles, and other fish swimming about is truly magical. At our last visit there this summer, the scuba divers were out and about -- they even helped with a wedding proposal which sent roars of cheers and applause throughout the restaurant!
Another little detail that makes dining there for dinner nice is that if we've returned to our resort for an afternoon break or have park-hopped and are then returning to Epcot for dinner, Coral Reef is not too far from the entrance. We don't have to walk and walk and walk to get to dinner.
PS - don't hesitate to throw this question out again for a second opinion!