Legend of Zelda: Ocarina of Time is hands down my favorite vidya game of all time. Virtually everything is perfect. The quest is epic, the graphics are beautiful (even if they are a bit dated), the dungeons and temples were designed by geniuses, it's chalk full of side-quests and minigames, and it has music composed by the one and only Koji Kondo. There's a reason it gets such good reception. If you haven't played it, you simply must.
And since this is a Sonic fan community, my favorite classic Sonic game is Sonic 3 & Knuckles, and my favorite modern game is Sonic Generations (though I don't really like any of the modern games that much).