It might be a weird answer, but Tales of Symphonia got me thinking through its immersive storyline. Let me explain.
At the time I played Tales of Sypmhonia, I thought a lot about religion and its role in society. Not that I have ever been (or will ever be) religious, but I encountered some people and events that got me thinking. ToS fitted perfectly into this line: the way I interpreted it, it challenged the way religion took hold of society. It showed that 'the light' and 'the good' are not always what they seem, and that religion is too often used as a means of enslavement. Hence the angels that turned out to be the main villains of the story. This was exactly the way I thought about the matter. It's also the reason I still hail ToS as the best game storyline ever created: it challenged a lot of social and moral issues, but never force-fed you an opinion.
I drink your milkshake.
