If you like strategy games, then I'd highly recommend the Advance Wars series. Both of the DS ones (Dual Strike and Days of Ruin) are really good and have many maps, plus the ability to create your own, but if you only wanted one, then you should probably go for Days of Ruin as it has WiFi capability, letting you battle other players online and download custom maps online too. Dual Strike does have multiple difficulty levels, but I don't know if Days of Ruin does (I haven't been able to complete the campaign yet, which is how the other difficulty levels were unlocked in Dual Strike).
Any other games I liked have already been mentioned.