France will be quite underwhelming until you're Enlightenment Age with Training Blessing. Unless you'd rather save crowns for something else, Britain/Korea will be much stronger until you have it.
As far as a "Britain vs. Korea" debate goes, most swear by the British Unique Unit, others prefer the Korean. A near maxed Industrial Age player in my alliance has tried both and it is of his opinion the Black Watch is better as the +1 Range provides much more survivability. However, the extra War Academy tactic of the Koreans, he says, can be very useful for 5-starring maxed/cheating players in World Wars.
Both the Black Watch and Righteous Rifleman are "God-tier" Unique Units right now, it's really a matter of personal preference.
You may also want to consider Germany, they got a rather significant buff in the last update that went largely unnoticed. An additional 5% (15% total) damage may not seem like much, but when you add that up for each individual Barracks, Factory and Mercenary unit (it's still not clear whether Air Units also receive the buff, they show the icon above them when deployed but CS claims that they don't), it ends up being quite powerful. Longer and faster rallies is not something to be overlooked either. Whether Germany is as strong as your other options is yet to be determined, due to the lack of end-game German players, but something to consider nontheless.
