obviously I wasn't literally in tears at any point, but the happy ending inspires that WAFF that I love so much, while the neutral ending scratches the melancholic itch of a child growing in an uncaring world that I usually take care of with Adventure Time or some of the more brutal episodes of Steven Universe.
The no mercy run is more classically horrific in a gothic/lovecraftian sense, and I don't have any compulsion to get that ending
I'll spoil the thing that affected me the most which was
All of the interactions with flowey, especially those if you kill Toriel then go back and save her. Like Bauglir said, I get the same creepy crawly feeling that some other games do when they break your immersion on purpose (i.e. Spec Ops the Line and Earthbound), and when Flowey started yelling at me for cheating I actually thought I was in a creepypasta for a good ten minutes.
TL;DR Come for the WAFF stay for the depth of gameplay
EDIT: also beating Undertale and Wolfenstein has made me want to beat more games. Not sure whether the next one should be Zone of the Enders or something like RE4 or Persona 4 that I haven't played in many moons
The thing that was in the spoilers got me.
I managed to make zero decisions that I regretted my first time around, so I never got the opportunity. The first time I actually saw any effects like that, I was well into the game. And still pretty impressed. Also, the boss that I was fighting was kickass anyway. And the boss after that did the same thing and was also incredibly kickass.
And the boss after ASGORE? Kickass. A lot of people were freaked out, but by that point I had sorta taken it for granted that the game breaks its graphics rules for stylistic purposes (the ball in Snowdin noticeably rotated on increments that were definitely not pixel-sized). I realized what point it was trying to make and found it too badass to really care. I managed to not die that fight, heh.
The pacifist final boss made me cry. I always have horrible "post-work sadness", where I'm immediately sad there isn't more stuff I can look at. Then the final boss had dialogue saying that he's not ready for this to end and i cried for the first time in like 5 years, it was impressive
but yes okay when it comes to undertale i will recommend something:
Do you like Earthbound's kind of humor? You'll probably like it.
Do you like JRPGs? You'll probably like it.
Do you like Homestuck? You'll probably like it.
Do you think that a game is trying too hard to make you laugh if it ever makes jokes? You'll probably hate it, but on the other hand you probably hate anything that's funny. (I only include this because it's a real criticism I've seen)
Do you not like it when a game comments on the way you play? Probably won't like it.
Do you not like happy things? Well, at least the game accommodates you there, but if you play as people recommend, it's gonna be happy (This is the most-quoted criticism I've seen).
Do you not get attached to characters at all? I wouldn't recommend it, then (this is the most
valid criticism I've seen).