Trying to pick which country is best in World Showcase at Epcot is like trying to pick the best candy bar ever made. It is a matter of personal opinion, and many of them have strenghths. Also, is it possible to talk to some one who has spent equal time in each country. They all vary in what they offer, and it would be impossible to spend a hour in each of them in the course of one day because the Showcase is not open for 11 hours each day.
I will tell you what I preferred. Norway has the best ride, USA has the best show/ presentation, Germany had outstanding food (I can't say best, because I didn't eat in every country, only 4 of them), Germany had my favorite shop, and UK had the best beer (I am biased towards Guiness).
My advice for someone new to the showcase is to gravitate towards the country that offers food that you know you like. It may turn out that Epcot serves the best version of that food you have ever eaten. The Biergarten in Germany definately had the best German food I have ever eaten. Also, if you like boat rides, make sure to head to Mexico and Norway. If you like American History, head to U.S. If you like movies, visit China, France, and Canada (only France has a sitdown theatre). If you like to shop for unique items from around the world, skip the movies and attractions, and stroll thorough each country hitting all of the shops. The gift shops at the front of the showcase feature a variety of items that depict the showcase as a whole. Each country has items that you can only find in their shops.
If you want to do everything I described, plan to spend 2 or 3 days at Epcot. We devoted 2 days to Epcot (remember, it also has Fututre World), and we still could have used at least another half day. Good luck and have fun.