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 :(
Mindustry. Played it on and off for years, abosultely love the game. Way too good consider the fact that it’s free, and when I picked it up I expected to get a couple of hours of fun out of it. But at this point it has lasted years. And with the occasional update, I think it will last even longer.
And it’s open source, too!