It's tough to go wrong in the World Showcase, in my opinion.
We like to go to places that we cannot get as easily at home, so places like Tutto Italia, Le Cellier and Nine Dragons are not as popular with our family as places such as Biergarten, Restaurant Marrakesh & Les Chefs de France/Bistro de Paris.
The one we're currently debating for our upcoming trip is Teppan Edo, a style that we can easily get here but that we really enjoy in Epcot.
That having been said, our favorites would probably be:
Biergarten (a lot of fun, and we both really like German food)
Teppan Edo
San Angel Inn (DW's favorite, largely for sentimental reasons, but we do all enjoy more authentic Mexican food, however, so this is a popular choice)
Restaurant Marrakesh