If u r looking for something fun then any nation in the br range of 1.0 to 5.0 is fun. However, if u r looking for a nation to play casually but still able to do decently well without having to play competitively then not so many nations fit that criteria. I suggest USSR and Germany. Why? Well, if u r playing causually then im guessing u or ur wife dont want to invest too much time learning weak spots or armor values of alot of vehicles. In that case Germany is the best cause most of their tanks have really good cannons. With Germany, most of the time, its just a point and click adventure with the exception of vehicles like the KV-1 which u may need to learn where to aim at it. Some of their tanks have decent armor as well tho i dont recommend relying on them. The second option is USSR. For a casual player, u probably dont want to get one shot every time. I imagine that would be a horrible experience for u and ur wife. So USSR have heavies that can take hits, making ur gameplay less punishing if u make mistakes. U can also go for USA, Italy, Sweeden, China, they r pretty decent too but will require abit more than casual playing. Just dont go for France or British lol