Honestly, I love this game. I remember playing it for the first time, and wow, the characters, the story and (DEFINITELY) the music - they all just captivate you in a game that you can't stop playing; you know you're in good hands when you first get to enjoy the comedic and lovable personalities that are displayed in Undertale, but also the engaging and interesting story that's at hand. I genuinely recommend this to people who appreciate RPG games, but you don't have to. I'm not much of an RPG maker fan but Undertale still became my favourite game. Call me biased if you'd like, but at least play it for yourself and see why I adore the story and characters so much.
A further reason that I recommend this game is because of the different paths and roles you can take. In Undertale, there are three options that you can choose from: the Pacifist, Neutral and Genocide routes. Depending on your preference, the story can change entirely - if there is anything that can teach you about the concept of consequences, Undertale will do the trick - and you can alter what the NPCs' opinions of you are with just one action. It has great game design, and although the art style may be simplistic, it adds to the list of qualities that this game has.
tl;dr: Undertale is a great video game. Despite how simple it may seem when you look at it, the concept is spectacular and it can generate such powerful emotions. I recommend it to RPG players and those who love a good story!