Undertale is an absolute treasure. This game is an incredible journey, and a brilliant and terrifying deconstruction of RPG tropes.
One thing that I greatly value from this game, that I find a little difficult to properly talk about, is the surprises/twists/revelations. Too many games nowadays, due to both less-than perfect writing and gamers' collective genre savvy, don't have enough surprises that really 'get' a player. Even if the game is fun, there aren't enough times where you get to go "OH WOW! I didn't see that coming!".
I greatly value the ability and opportunity while playing Undertale to be surprised by a game, whether in the form of a hilarious punch-line, a shocking revelation that makes sense the more you think about it, or the sudden formula shift that turns a fight upside-down and makes me panic super hard. I thought the game was super strong in this regard... until I reached The Throne Room, at which point, the game decided to become the most twisty, surprising, revelation-filled 30 or so minutes that I have ever experienced in a video game.
I counted, and in the time between first meeting Asgore and landing my first attack on him, there were SEVEN revelations I did not see coming, and some of them took me stepping away from the game to realize.
So, most important thing: Holy moly mother of Toriel's Butterscotch Cinnamon Pie... Omega Flowey the scariest final boss in any video game I have ever played. The phrase "Nightmare Fuel" is thrown around a lot, but I would honestly not be too surprised if Omega Flowey actually sparked some real rough nights for players. The use of bad clip art (or whatever that stuff was) and unfittingly fluid movement was so effectively grotesque, unnatural, and aesthetically contrasting that it made it into a horrifying abomination.
I knew I was gonna spare Flowey no matter what, but I honestly considered the other option more than I expected to.
Edit: Also, the forced game shutoff before Omega Flowey? It totally got me. I was starring at my desktop for like two minutes straight, slack-jawed in utter shock and horror, before I even thought to start up the game again.