Mine was Donkey Kong N64. I was about 2 at the time and I just watched my brother play it in front of me and when he would go to the bathroom, I would steal the controller and move around randomly. It was fun.
But for first Legend of Zelda game, it would be Wind Waker. I was 4 when my brother got it and I would just watch him play it and when he would go to the bathroom, I would steal the controller (again, I had to do it for a while before my brother would let me play) and I would play for him until he got back. He would get really mad if he found out I stole the controller and played for him, but when he sees that I defeated a hard monster for him or solve a puzzle for him, he would forgive me and I wasn't until 5 until he would let me play the whole thing without getting mad and he would watch me in awe as I beat really hard bosses for him. (He's a Zelda noob, so he was really amazed by my talent)