IGN reviewed Undertale - and it got a perfect 10:

Undertale Review

Undertale’s clever understanding of the RPG mindset and fantastic writing make it an unforgettable experience unique to games.

I finished my first playthrough of Undertale in stunned silence. My journey had begun with dumb puns and silly puzzles, but the end affected me in a way I never expected. That’s kind of Undertale’s specialty — playing with our expectations of what an RPG should be, subverting them, and using them to drive a story unique to what games can do. Its strong writing, integration of gameplay with storytelling, and acute understanding of its audience all build to something that surprises at every turn.

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The Verdict

It's hard to express just how much I adore Undertale without spoiling anything significant, but that's what I love about it. It tells its story in such a dynamic way, and with such a great understanding of the RPG player’s mindset, that it couldn’t have been told in any other way. It's a masterfully crafted experience that I won't forget any time soon.

Score: 10/10 It might not be pretty, but Undertale is absolutely a work of art.