I used to be a Nintendo fan but now I've grown up and Nintendo is just a hobby on the side of mature gaming console.

Can't really say it was one game, but the collective libraries of NES and SNES that made me love Nintendo

For me it's definitely Pokemon. I played these games my whole childhood, sometimes even nowadays.
Honorable mentions would be Zelda Windwaker, StarFox Adventures and Super Smash Bros Melee.

zelda on nes

Goldeneye, which made me pick up a N64, which made me a fan of Zelda.

Zelda (NES). Really I stay with Nintendo as my favorite because they focus on gameplay, controls and level design. I'm not into cinematics and everything having to be bland open world.

For me it was Starfox on the SNES.

It's epic fight to turn back an entire enemy army, overcome massive and powerful bosses, and reclaim a conquered solar system of exotic alien planets enthralled my space-and-science-fiction-loving brain, it was the epitome of cool.

Even as a kid I'd already started keeping track of what studios and directors made the films I liked, and Starfox burned "Nintendo" into my mind as a company that delivered grand adventures unlike anything I'd experienced before.

I love Nintendo since Super Mario Bros and Duck Hunt, But Zelda 1 was the one that made me a fan.

I've become a mainly Nintendo fan three times. I think from Mario onward, I've always been a Nintendo fan to an extent, but I've also been mainly a Square/Rare fan and a Sega fan. And Square going to PSX took me with them, especially by the end of the N64 era with crazy Yamauchi's comments and ridiculous pride.

1. Super Mario Bros, made me a huge Nintendo fan. But I became a Sega fan in the 90s, I also caught the Sonic the Hedgehog bug.
2. Secret of Mana/DKC I think the Square and Rare combo really did a lot to win me back in the mid-1990s after a brief stint as a Sega fan (it felt a lot longer than 3 years in my memory, but that's about all it was).
3. Animal Crossing/Wii Sports - while I still was a fan during the N64 era, I just about all but forgot about Nintendo after Conker, but I was on my way out by the time of GE007/Mario Kart as becoming much more interested in the PSX (mainly because of Square, Capcom, and Konami games)... Animal Crossing made me look back at Nintendo again at the end of the Gamecube era, and then a year or so later when Wild World came out on DS, I was fully hooked on their handhelds - it was a different experience, but I enjoyed both for different reasons - AC Cube was more of a community game while AC DS really made AI-social creation a big deal, carving out your own reality. AC Cube also came out on the eve of all the Revolution/Wii hype - and it was Wii Sports and the year 1 Wii lineup that brought me back full onboard with Nintendo.

While I wasn't on board with Wii U, I still played the crap out of 3DS, and Sony failed to win me back as, IMO, they are not the same company they were with the PSX and PS2. Playstation has become a full-fledged plug-into-TV PC gaming machine. So, I didn't need Switch to win me back.

I think Super Mario World on SNES was what made me first like NIntendo, then goofy troopers and DKC.