Pokémon Ruby
While I think I owned Pokémon Ruby briefly as a kid, I never really played it. Pokémon FireRed was more my jam, and by the time I got my hands on Ruby, I already had Pokémon Emerald so there wasn't much point in me playing it.
With that said, after I beat FireRed, I took up "Professor Oak's Challenge" in Ruby. But instead of playing the game on actual hardware, I played this with an emulator on my computer. This means I was holding down the speed up button the entire time. Despite that, it took a little over a month to beat the game.
What kinda sucks is that this game lacks anything after you beat it. It's pretty barren all things consider and while it was nice to finally play through it proper, there wasn't much else to do once I was done with it. There are only 8 version exclusives, so I knew I wasn't going to be missing out on much, but I decided to play Sapphire after beating Ruby. I'm kinda happy I didn't touch this too much as a kid, because I probably wouldn't be able to appreciate it for what it is. I spent 113 hours in-game-time completing Ruby so not horrible all things considered.