Undertale is a flawless game through and through.
The game's story is amazing, with each of the different endings playing the story out in a different way.
The game's combat is beautiful. The system of blue and orange attacks, the way your heart can change colors, it all just works flawlessly. Even though it gets very hard coughsanscough it wasn't ever frustrating, as each and every encounter has mechanics that are an absolute blast to learn and master. It is the only game that figured out how to make turn-based combat fun, and holy hell is it a blast.
The characters are AMAZING. This is the only RPG whose characters aren't obnoxious and annoying. Each and every character has so much personality and character, even just the random side enemies that you can encounter randomly. Right off the bat you meet one of the most interesting antagonists of any game, and also meet one of the funniest and best characters of all time shortly afterwards. Every character was so likable and enjoyable. (Sans is the best character and you can fight me)
The world is a lot of fun to explore and although small, the world still feels more alive than that of any other RPG I've ever played.
Overall, 11/10, would definitely recommend