I say Ocarina of Time. This might be a silly reason, but it was the first game I ever played that showed me that a game could be more than just repitive racing and jumping on floating blocks--a game could be an entire world, memorable characters, and a story and won't let go of you. It was also the first game to scare me sh**less. Damn Shadow Temple...I was way too young to be battling undead things. I'm still freaked out by it.