Which country is a good one for beginners?
USA is probably the best. It only has to fight a couple of times - both times against enemies which it can defeat without too much effort. The Mexicans will fold like a deck of cards, and you can slice through the CSA like butter. The Civil War won't take you 4 years to win, more like 4 months.
However, if you want to fight more wars, the USA, unlike some other countries, can handle it. You have the resources to defeat Britain and its allies on land, although you may have a problem projecting power off continent due to the Royal Navy. But without too much trouble, you can occupy all of Britain's North American territory and allies such as Canada if they exist. And if you win, even conquering all of Canada and all of Mexico, the other Great Powers will still be too afraid of you to attack in retaliation.
The #1 complaint in the Victoria community is being unable to attract enough immigration to their country. The reason for that is the USA is stealing all those potential immigrants. If you're the USA, you will benefit from having all your factories fully staffed and having a large, gradual increase in population and territory throughout the game, no matter what you do or how good or bad a player you are.
If you try to play Brazil, for example, you won't get immigration. You might get a little bit, but it will be very expensive and complicated to get even that little bit. And even if you do everything possible, it's not even all up to you - you might still never get any immigrants if there are not enough push factors from the countries of Europe and China.
Trying to play any other American-continental country besides the USA is an exercise in futility; you will never be able to compete with the USA because of their immigration bonus. If you try to conquer foreign territory to increase your population and resources, this will anger the Great Powers who will immediately attack you in retaliation. You will not be able to defend against them, and you will lose.
There are some European countries which are kind of suitable for a beginner. If you play Prussia, France or Russia, it will be difficult to screw up so badly that you lose Great Power status. Those countries have a lot of economic activity and many events and several automatic wars, so the gameplay is interesting.
Britain might be too easy because it's basically invincible. But because its empire is so large, managing it might be too complicated for the beginner.
If you want a quiet game, you can choose one of the European minors such as the Netherlands, Belgium, or Sweden. They don't have to fight much or at all. Your goal with one of these would be to end the game as a Great Power. You probably wouldn't want to try to dominate by conquering other civilized countries, because you would quickly be reduced to nothing by the other Great Powers.
Japan is a fun country. The only problem is the boredom waiting 30 years for the Meiji Restoration. Before that, there's not much you can do except watch.
Sardinia-Piedmont is a challenge but if you succeed, you get to become Italy. I don't recommend it for a beginner.
Other countries will probably be too frustrating or boring to play unless you know what you're doing - ie. Egypt, Spain, the other Italians.