Undertale is my favourite game . . . that I've never played. I love the story/themes of Undertale a lot (which I experienced through a Let'sPlay years ago). What I love about Undertale is that every element of the game design is in service of it's themes & story. The medium of gaming itself is an integral part of it's story, and I love that.
Eventually I will play Undertale myself, maybe even soon. A little annoyed to read Nintendo Switch has an additional boss, since I've actually owned the game for the PS4 since it first came out on that system. I knew I wanted to play it myself some day and I really wanted that locket with a music box that plays, the main theme I think. Because of course, the best thing about Undertale is still probably it's soundtrack. Absolutely phenomenal music, one of the best in gaming history imo.