I'm pretty sure it was Hamtaro: Ham-Hams Unite for the gameboy, but I still play that game and I've beaten it a least five times (I obviously have nothing worthwhile to do with my time) If it wasn't that, it was probably the first Pikmin game on the gamecube.
I want to say the original Legend of Zelda (NES). I don't think the visual novel type games I've beaten count. They're technically completed 100% but it's different than completing a game with collectibles and things, like Zelda.
Uh, I think it was some sort of Barbie game that I got when I first got my Gameboy Color. Other than that, I haven't really 100% any other game. I've beaten the stories (like in Pokemon), but never really gotten out to filling out everything (like filling out the Pokedex).
And all of the other games I play either don't have an ending and are sandbox type games or don't have a way to 100% them.
That honor belongs to Kirby and the Amazing Mirror. So much fun to explore the levels to find all the rooms and treasure chests. I want another Kirby game like this with the metroid-vania aspect of free range exploration.
First game I ever 100% was my 3rd playthrough of Sonic the Hedgehog 3 + Knuckles.
Got all the Chaos Emeralds + Special Emeralds and recieved Hyper Sonic.
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