Fantastic game, the less you know going in, the better. Undertale is a complete teardown of every typical RPG mechanic, and weaves it all into an incredible story. The soundtrack is also excellent, hard to believe the entire experience was created by a single person. Highly recommended and one of my favorite games ever.

Update after playing through all 3 endings again: still 10/10

Update 11/25/2018: Still an amazing game, now play the follow up DELTARUNE