i think one of the best momments for me was way back in 2017 when i was making my first playthrough on the neutral route. I had little to almost no computer knowledge back then so when flowey simply crashed my game i got so scared that i didn't touch my save for at least an entire day
art source: https://www.tumblr.com/9twoeight
I (like many others) accidentally killed Toriel and reloaded the game. When Flowey KNEW about it, I was totally freaked!
on my first playthrough i killed papyrus and then got so scared by undyne that i didn't kill anyone after finishing the fight with her.
Reading the ancient glyphs in Waterfall with the beautiful music playing in the background and the castle view section with monsters kid. Imo, that was one of the most immersive experiences in Undertale.
Also, the moster tale/ new home segment. The music, the plot buildup... everything was awesome. And the last froggit you encounter which says "You're going to be free." - that was my favorite Undertale moment.
