TL;DR: Undertale is a great game, but not due to complex game-mechanics but rather because it accepts that it is a video game and uses this medium to deliver a truly unique experience that constantly subverts your expectations to play with you, just as much as you try play with it.


Since this should be a review, here are some bullet-points:

• Pixel Graphic with sometimes less amazing parts, sometimes areas with a lovable attention to detail
  
• A witty sense of comedy alternating with emotional, serious or deep sections
  
• Combat admittedly gets repetitive due to the nature of the Battle RPGs
  
• OST is amazing (Undertale wears the "Great Soundtrack" tag for a reason) and its songs capture and recall the respective feelings

But this kind of review wouldn't do justice to Undertale, for it has one big strength when compared to other games: It isn't just an interactive movie, it recognizes the player as a player and therefore as an unavoidable variable.

When I gave into the Undertale-hype, I started the game with high expectations. It not only surpassed these, but I have spent way too much time watching and reading about Undertale theories, OST, remixes and other content online. The toxic part of the community has gotten a lot of discredit, but the content-creating-part is astonishing.
So I started the game, knowing that it would try to play tricks on me; so naturally, I challenged its bounds. But the creator, Toby Fox, created an experience where you are either outsmarted by the game or, if you think you can outsmart the game, find yourself surprised because your choice was anticipated and you are now being toyed with non-the-less.
A UI is usually just a display for information, but in Undertale, its as much a part of the game as any other object.
A Game usually defines certain rules and mechanics that are tied to the gameplay, but in Undertale, these are frequently changed, broken, inverted or flat-out thrown out of the window, leaving you with a memorable moment every (boss-)time.