I had not played Undertale until 2017 when it arrived on PS4 because I didn't own a computer in 2015.
When I did play it I automatically fell in love.
Undertale sucks you in with it's ability to make you laugh, cry, scream, panic, question wheater your morals are worth something and hurt your brain.
Undertale's flaws are very small, the main one being the graphics (Not that they matter anyway, it's better this way because it doesn't take much space.
The gameplay is fun, with the bullet hell combat being very unique and keeping the gameplay fun.
The characters are fun and quirky. Sans and Papyrus are a riot and they are all interesting in their own way.
The music is on my phone and will be listened by me until the day I die. It's just perfect music and there's a song for every type of moment.
And of course there's the fact that you get punished for killing. It's a weird idea but one that's pulled off brilliantly.
I just love Undertale. Buy it, play it, love it.