The very first time I fully completed was Super Mario Advance 4: Super Mario Bros. 3 on Game Boy Advance well before the Nintendo DS was ever released. All of the levels are completed and the score is maxed out in SMB3. And I don't count the exclusive e-Reader levels, because they were nigh impossible to find in Canada to begin with (at least in Qu?bec). Also, I ignore the Mario Bros. game included on every Super Mario Advance game.
If the above game doesn't apply, then I would consider Mario Kart DS as my first fully completed game. All staff ghosts are beaten, every single cup and mission have 3 stars on it, every character and kart are unlocked, along with the ability to choose any kart with any character. I started the game on December 24th 2005, and it took me 10 months to 100% it. Oh man, this was one my proudest moments I've ever witnessed in my video gaming career.
The first game I completed without guides was Kirby: Tilt and Tumble for the Game Boy Color. It was a really cool game, especially because all I had were Game Boy Color games.
I don't think I've ever 100% a game before. I tried to do it with FFX-2 with the help of a game guide, but some of the side quests were really hard so I got frustrated and gave up.
Props to anyone who has the patience to entirely complete a game though.
I'm not sure if I have ever 100% a game before as I usually just get bored of it and restart a new game. The one that I did beat all the way though for sure was Zelda a Link Between Worlds. That game was fairly quick and sorta easy though so that wasn't really tok hard. I think I might have 100% pokemon emerald way back cause I remember playing that game for forever, oh and maybe super smash bros melee, I think I collected everything on that game...
I'm not too sure which was my first one I 100% though, but I have definitely not 100% many games compared to most people xd
The first game that I completed 100% on was Mischief Makers on the Nintendo 64. That was such a fun and underrated game, I used to play it all the time but I was stuck on it for ages when I was younger.
pokemon silver maybe...? I mean I beat the game, but I never caught all the pokemon or anything, so idk if I could consider that 100% complete... X's the first game where I actually complete the pokedex.