For learning purposes just play classic GOW, EF and nothing else. If you want to have fun play other built-in maps, but they won't really give you any insight into essential mechanics that can't already be found on classic GOW. You will still find yourself on occasion playing maps that have a certain uniqueness to them like water maps, nwtr, and ones that have more gold scarcity than you will be used to on GOW, but once you understand GOW for the most part, you will be able to pick up those maps with just a few easy adjustments.
GOW BNE is a relatively similar map to classic GOW, and the way it is played has a lot of similarities as well as long as you don't change the game speed. Hardly anyone plays it though, so you might as well be practicing on the map that people do play so you can concentrate completely on learning the tactics and maneuvers that are exclusive to classic GOW.
Yamon also gave you a great piece of advice - focus on playing people who are better than you. If you don't make this a conscious effort, you will probably find yourself unknowningly resorting to almost only playing people you can contend with and it will slow your learning process.
