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 :(
I was a Bridge Commander fanatic, picked it up at launch, played way too much of it, modded til it broke dozens of times, participated in the forums, even wrote a GeoCities page where I put out tips for the more challenging campaign missions and did write-ups on every playable ship in the game. Good times.
RoN is also awesome, and is on sale on Steam! Purchased!