For me it's easily Twilight Princess. I guess it can be classified as nostalgia now as it is nearly 8 years old.
Besides playing a few games in the series prior, and owning Phantom Hourglass, this game holds true sentimental value as it was the first 3D title that I fully owned. Sure I had PH a few months before, and that game was great; however you get a different feeling when you play, what I would call, a true home console release.
It's the only title in the series which I can pin point every moment and match them with a certain emotion when playing. Every track, cut scene, boss fight, dungeon all take me back to that first time playing. I still remember the day I got it, taking my time and absorbing the surroundings of the first few moments of the game, which actually took me 4 hours to complete - party due to me being slightly stupid, but mostly due to the fact that I was just taking in the surroundings. It was like this the whole way through to be honest, I appreciated every slight detail of the game and enjoyed every moment which is somewhat of a rarity with any game (only a small handful have that accolade). Of course now I look back and see it's not as flawless as I originally viewed it, but that first playthrough still stays etched in my mind.