And by that, I don't imply that this Route should be in any way pleasant to pursue or accomplish, given the game's morals, nevertheless it was a mixed bag.
Best:
-- "sparing" Sans and what follows after. It wasn't too hard to guess the outcome, but the comic reflief made it totally worth it. I laughed the roof off.
Good:
-- I sort of feel the main narration behind the events is the most fulfilling (and least ambiguous) of the three routes, given how Flowey "opens up" or the new meaning given to the entire game you're playing (or performing), even if it comes at the price of going down some very dark paths
-- Sans' dangerous nature and awareness hidden behind all that laidback facade and goofskull. This could count in general, but Genocide improves upon it, too. If there's one thing all Routes (well, perhaps except the Pacifist's very end section) make me feel, it's that Sans is never really my friend or companion throughout the game, the way I end up bonding with all the other main characters; he is much more aware of my nature, what I can become etc., and is invested with powers of understanding past the game's walkthrough. The part that makes him compelling as a final boss in Genocide is the you-asked-for-this-kiddo resolution. That being said, check Neutral as well.
-- if good should equate with "haunting", given the nature of this Route, then I suppose the most haunting detail was the subtle hints at how a malevolent force gradually takes over me. Stuff like dialogue lines changed in red or the character advancing forward without my control in certain situations definitely did the trick to unsettle me.
Neutral:
-- sadly, I ended up spoiling myself too much with synopsis and such, for the Genocide ending to have the purest, most stark effect on me; that's not to say it isn't designed, in itself, in a compelling way
-- also on Sans, I'm less sure why, with him being so cognizant (and hinting at this explicitly, as soon as I enter Snowdin), he allows me, the player, to go ahead with all the destruction required to complete this route; it feels slightly gimmicky that he's supposed to take his stand at such a late, point-of-no-return phase. I think the Leaderless Ending in Neutral was more poignant in revealing his grief that him going easy has led to so much bad stuff happening.
Disappointing:
-- I was sort of expecting, regardless of the minimum kill count necessary, to irreversibly damage separate screens of a stage as soon as I confronted and killed every monster that can be encountered in that particular screen. But nope, as soon as you reach the quota, the entire stage is barren. Sounds slightly illogical and also not really rewarding.
-- the "but nobody came" screens are automatically triggered, once you've crossed the Rubicon in each stage; I suppose this is meant to torment your conscience, but it's in fact a bit too annoying as a mechanism. Of course, most of the times you're expected to complete the quota near the end of the stage, but one time I achieved that deep inside the Snowdin Forest and, oh boy, was making my way to the town fun-fun-fun afterwards... NOT
-- the fact that Genocide =/= Hard, except for two specific events. Again, I understand the game is designed to not make me feel good or excited about anything I'm doing down this path and that this game universe is created so that almost every monster is less powerful (and determined) than me as a human/player, but I still wanted to be way more challenged than I was. Several one-shot twists are probably the most disappointing aspect of this Route, because it only makes me go back other Routes, to at least dodge some shit for a few minutes with those characters, and the rest of the encounters are just dire. So basically the battle part of Genocide goes down to just two main challenges - granted, the first being yeah-this-will-probably-take-me-20-attempts tier, and the latter... well, what can I say... I asked for it.