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 :(
Boktai. A trilogy of GBA games about a vampire hunter who uses a solar gun to fry the undead. The cartridge has a UV sensor to detect actual sunlight to charge up the gun. It’s such a silly gimmick but it’s used really well.
Sadly the third game never got localized, and although a translation patch does exist it’s just not the same without the original hardware. There’s a fourth game on DS which did get localized, but they rebranded it as Lunar Knights, excised most references to the original trilogy, and even cut a good chunk of content. It also ends on a sequel hook that will never get resolved.
It’s sad to me that we’ll never see games this experimental ever again.