Mine has to be Dragon Quest: Rocket Slime, a DS spin off of the Dragon Quest series that sees you playing as a slime operating a tank and rescuing the people from your town. You run around the overworld, collecting items to use as ammunition and saving money to upgrade your tank. The art and music are just as great as you'd expect from the Dragon Quest series. It made fantastic use of the DS's dual screens. It's also written for a younger audience, so a lot of it is just really silly and fun! Try it out for sure, I'm so sad there's no sequel :(
Profound story, excellent sound design, amazing soundtrack, insanely well designed combat system. It's just an incredible game. It has more content, depth, and attention to detail than most AAA games these days.
If you're picking it up, get the DLC as well! It's not just DLC, it's the conclusion of the game's story!