I'd like to say TWEWY like everyone in the thread, but it was a big disappointment for me, it was linear and flawed(at least for the battle system) despite good reviews.
My personal favourite will have to be Legend of Zelda: Phantom Hourglass, not exactly the OMGWTFITSTHEBESTOHITSSOFUCKINGREVOLUTIONARY game, but it's pretty polished for me, it's one of the first DS games I got too, so it may possibly be nostalgia, but ti's definitely the game I enjoyed the most on the DS.
Another game that is probably on par with Zelda is Dragon Quest Heroes: Rocket Slime, just pure fun and creative gameplay, love the presentation and all. The tank battles were just too addictive.
Ahhh, nostalgia