As @Colt45-554 mentions, Global 40 2nd and Balanced Mod 3 of it are by far the most popular here on A&A org with their own league that is very active playing on TripleA: https://www.axisandallies.org/forums/category/29/league. After that its kind of a assortment of mostly TripleA custom maps like Napoleonic Empires, Domination 1914 NML, World at War (WaW), LOTR, etc.
On the Online TripleA lobby, the most popular maps are probably in this order: revised (v2), Global 40, World at War (WaW), New World Order (NWO), Total World War, Big World, and anniversary (v3).
As @Panther mentions, most of the custom TripleA maps don’t exist as a physical board game. So the best way to learn is play against the AI.
If you want something similar to classic and like playing online then revised (v2) is probably your best bet. If you prefer playing PBF/PBEM then Global 40 is your best bet but it is more complex and a longer game. But the best thing to do is try a bunch of different maps out to see what you like.