Hmm, I'm surprised no one else has replied. I certainly have not played every country. I can say I've been Quebec, Argentina, Brazil, Russia, Ghana, Libya, F-A, India, and China. Of all of them, I like Quebec and Ghana the most.
I like these two nations because they are fairly compact, have lots of centers that they can usually go after, are bordering on a major ocean space (that you usually end up controlling) and have two close neighbors you can choose for the easy stab (as opposed to Argentina, Oz, India, etc.)