oh god I loved so much of it. I loved the characters first of all - I think the moment I decided I was absolutely in love with the game was when I realised I could ask a skeleton on a date. The thing really kicks off during Snowdin - Sans & Papyrus are a combination for life, but it just gets better and better. I absolutely
love have a small crush on Undyne, she is incredible. I like how so many of the levels are set up as horror/suspense - the approach to Snowdin Forest, the shadowy spear-throwing sections in Waterfall - and then the characters behind them turn out to be
such goofballs. Also Alphys is adorable

. I think Papyrus, Undyne, and Alphys are my favourites - especially Papyrus & Undyne's phone calls, those are absolute
gems, I love how they're just great friends more than anything.
But I also really came to love the gameplay too. I like how each level is quite unique and you never feel that you're treading over the same old ground, or that you're never grinding. The puzzles are all different - and as someone who is
bad at puzzles I appreciated how simple they were. I also think the level design is so pretty and I like how the colour palette for each is so strikingly different. It's a really fun game to
play and it never feels like it's dragging. Also the True Labs really did spook me out, it was past midnight when I was finishing that up so eeeek.
Also, it sucked me in with feels. I never expected to care much, Undertale seemed like a cute, quirky game with a few really cool characters, but on the pacifist ending I was like NO FLOWEY YOU CAN'T DO THIS I LOVE THESE PEOPLE. I cried during the Asriel fight. By that point I had thoroughly spoiled myself because I didn't expect to be so invested... still cried, though.
In terms of playthroughs, I did a neutral playthrough where I killed a bunch of monsters during the ruins, spared Toriel (I refused to kill her, cheated and googled how), and then got chided by Flowey for killing a bunch of monsters with families. I felt so guilty that I played the rest of the game as a pacifist, at a grand total of level 3. Made the Hotlabs really quite difficult, although it meant replaying the pacifist run was a walk in the park.
I think I might do what you recommend with the no mercy/genocide run. I expect it to have more horror elements than usual, and I can't stomach horror games. I find films/books totally fine, but there's something about the interactive element, and usually the fact that I play alone, that really spooks me. Also, it'll screw up a pacifist ending, so it's probably one of the few games I genuinely don't ever want to 100%.