I believe The Beatles: Rock Band came the closest to being perfect. Eveything about that game was just beautifully done and the only things missing was Pro Drums, an option for Keys, and a few more Beatle songs (Hey Jude, Strawberry Fields Forever, Yesterday etc. etc.)
My answer to this is always Portal, the first one. It was so unexpected and so, so good. Nothing in gaming before or since has been that magical of an experience for me. Maybe early Pokemon, when my little kid eyes were opening to what gaming could be. But there’s just something special about Portal. Such a concise, perfect little game.
Portal’s beauty is in it’s shortness. Perfectly paced, it takes the basic idea, plays with it, twists it, and finishes before it can overstay its welcome. In and out in an afternoon.