Ocarina of time, no contest. I have so many memories of playing that game when I was 7, and have replayed it so many time!! And it really does seem to still hold up today, I finally talked my Girlfriend in to playing it on the virtual console and she's loving it!! She started this morning to play for "a little bit" and has now been playing for about 4 hours, she can't put it down.
Normally I hate to watch other people play games, but I've been sat beside her the whole time (Not helping - that would ruin it) watching her play, and it's fascinating watching her figure things out for the first time. And it brings back so many memories for me.
Other possible contenders include...
Mario 64
Super Mario Bros 3
Tie fighter!!
Pokemon blue
Rouge Squadron
Timesplitters 2
Super Metroid
You can't go wrong with any of those games!!