Undertale is one of my current hyperfixations!!! I love it very very much and it is very important to me! I also really enjoy Deltarune.
One of the things I enjoy most about Undertale is just the silly jokes and gimmicks littered throughout it, like the artifact that gives you dog salad, or the mushroom that does the mushroom dance. There's little details, like how the mushroom dance changes depending on how many monsters you've killed, or the phone call at the end of the game, where there's loads of different endings you can get. The detail of Sans counting how many times you've died in battle, to Asgore weakening when you eat Toriel's pie. It's the little things that make it a great game.
The soundtrack is a masterpiece in itself. Megalovania is a great song, and while a lot of people say it's overrated, I think it's so cool how it's evolved over the years. Toby has had it in three different projects, and the differences between them are so cool. Death by Glamour, in my opinion, is CRIMINALLY underrated. That song is an absolute banger.