This Godzilla game
It's a 2D puzzle-platformer I played almost a decade after its release, back when I couldn't even say my own name properly lol. I'd spend a few hours each day until the batteries run dry, then recharge it so I could play the next day. Will never forget the day I solved that final stage, and how I replayed it over and over again looking for other solutions. Man, those good ol' Game Boy days.