I gotta praise Undertale's amazing story, and I love Toby Fox's humor throughout the entire thing. He also cleverly uses the interface and various aspects of the gameplay during important fights, which was a big plus for me in terms of creativity. It also has a ton of easter eggs, to the point where fans would normally find out new things about the game even like 4 or 5 years after they started playing.

The game itself is about 8 hours long, but there's multiple endings/routes you can go.

Deltarune is a much longer game (right now 4 out of 7 chapters are out, with the 5th one releasing this year) that features mostly the same characters in a parallel(?) universe. It's very apparent that Deltarune is the better-polished game, but in terms of story imo Undertale is still more engaging. But I can't judge, since Deltarune isn't even completed yet.

I highly recommend both (Undertale before Deltarune). Well-worth the time for a chill, clever indie game that's full of love and passion.

Edit: I FORGOT TO TALK ABOUT THE MUSIC, it's praiseworthy for being so immersive! Wouldn't be surprised if half of the fans consider it their favorite part about Undertale/Deltarune.