There are too many to name! If I just had to choose (thankfully I don't), my must-do's would be the escargot in France and the lamb chop in Australia. You will not be able to do the entire World Showcase in a day unless you are really hungry or have nothing else planned from 11 a.m. to 9 p.m., so look at the map and determine where you hope to start and stop.
Nights are always busier than afternoons and weekends busier than nights. Most of the bites are share-able for 2 people, but not 3, and some you won't want to share at all.
Disney Food Blog has pictures of all the food up on their website, along with the prices for 2013. I suggest looking their to get an idea of what you might like to try.