The World Showcase is our favorite spot in WDW! DH and I like to grab a glass of champagne in France for me and a beer in England for him, then watch musical sets of The British Invasion (Beatles tribute band in the UK), Off Kilter (Celtic rock in Canada) and Mo' Rockin' (World Music plus a bellydancer! in Morocco).
We like to eat dinner at a table service restaurant (so far we've eaten at Rose & Crown about 4 times, Mexico twice, Morocco once, Le Cellier once, and Chefs de France once) or, during the Food and Wine Festival, we snack from booth to booth.
Others have mentioned many of my favorites already, but I don't think anybody mentioned the Tin Toy Museum in Japan--we just discovered it about a year ago & it's very cool!
Don't miss the acts outside of France, and as somebody else mentioned, make sure you pick up a copy of the weekly entertainment times when you enter the park. The times guide is only available at the entrances to Epcot, as far as I can tell.