Probably My Favorite Pinball Video Game
This is by far the game I spent the most time playing on the Virtual Boy. Pinball was a great idea for the extinct Virtual Boy system and I enjoyed the space aspect of it.
Each stage gave you a different challenge and trying to build up your score was addicting. For the most part the game wasn't designed to screw you by leaving areas where you'd be sure to lose your puck which is nice. You can also get defense upgrades by doing specific tasks in each stage. There are mini-games on each stage where you can rack up points by the millions after you hit stars. Heck, there's even a little "Metroid" thrown in which was a nice touch.
An under appreciated game on a dead system.