I know you said it already, but I think Advanced Wars is easily the best portable strategy series ever. My favorite on the DS though is Days of Ruin (its called something else outside of the US), not Duel Strike.
While a lot of people didn't like them coming up with an entirely new story, feeling and mechanics, I personally think it is a lot better game. While part of me misses the old COs, I'm glad they got rid of the "lets play war and eat ice cream afterwords!" mentality. It's also a bit darker (at least a fourth of the COs die in the story), though some of the series signature humor remains if you look for it.
Mechanics wise the battle formula's have been completely rewritten, the CO system was entirely changed, and they finally included a map editor that can do almost anything. It's a lot harder to get 1 hit kills now, and units now gain experience when they kill enemies, which provides some modest stat boosts and gives you a reason to keep them alive. CO's no longer effect all of their units, you have to put them directly in a unit, and they'll give a boost to units nearby. All COs now only have 1 CO power (actually 2 have none), but it's more equivalent to the super powers in Duel Strike. The extremely unbalanced duel strikes have obviously been removed from the previous title. Another thing worth mentioning is online play, which is decent if you don't get a game where someone hacks, but they are becoming hard to find lately. Setting up matches on websites is still alive and well though.
In other words, you should get it once you are done with Duel Strike, especially if you can find it used.
Other than that, there is Fire Emblem, but I wasn't all that impressed with it. I guess it's a remake of the original, but I found the GBA titles more entertaining honestly, and the series consistent lack of a free play mode seriously limits its potential in my opinion. Especially now that AW:Days of Ruin has a good map editor and can save up to 50 custom maps. A lot of people seem to love Fire Emblem though, so maybe I'm just the odd one out here.
There was an Age of Empires game for the DS as well that got decent reviews, but I haven't played it. I'll probably think of more later but that's where I will end this list.
