What's your main focus, war? British, German, Korean are equally great armies.
Medals and resources? French and British are the best. French fast training especially on TB is really something, allowing you to loot more per hour. British 15% extra loot is very nice as well.
Chinese comes after the 3 top nations, it's good in wars where you always use mercs. Roman comes last, everyone i know who switched from roman to German, korea, british, reported a better performance in both areas (war and mp). Better off French then Roman, with 8 less troops but 3 times faster TB and better tanks.
Japanese and Greeks are bad, really really really bad. Greeks have like 50min free building time over a 12days upgrade? and Light tanks aren't suppose to do high damage, they're suppose to take some beating while the others are doing all the damage and even with the extra 25% they're still not good against walls and HTs and get shattered by AT like other tanks.
Japanese doesn't really have anything to offer for war nor mp.
I used japanese, french, german. Japanese was a disaster, French is nice because of the fast TB. German army is farrrrr more superior then french army but i'm always stuck on oil since i don't play more then 2 hours per day and don't buy crowns, beyond 2 hours i guess you're good.