Saving Miia by ramming a boat in the middle of a large lake. With a swan pedal boat, to boot.
He bites right through Rachnera's silk! A spider web of that size would be stronger than steel! Course it was a single strand of silk, and cutting through webbing is different from actually breaking it, but it still surprised Rachnera.
Kimihito shows his badass credentials again by calling out Ren on her motives, stating everything about her that made him refuse letting Rachnera go with her. She didn't get the message, but that was still pretty admirable out of him. Rachnera was quite moved too, although she didn't show it until she got drunk on coffee.
Also, there was an accidental Take That! to her as Kimihito had just come home from shopping and had brought something to drink for all the girls, and his gesturing during offering them the drinks was basically saying that "This guy can deal with half a dozen other girls than Rachnee on his own, and you're still convinced that you would make a great host to her?". Of course, she didn't notice that part, but the internet did, and Kimihito's popularity rose.
Chapter 27 had quite a lot. Luring Miia's mom into a refrigerated storage unit, withstanding the aphrodisiac she drugged him with by giving himself a cold shower and finally dragging two unconscious lamias to an onsen where they could recovernote For the record, an 8 meter anaconda (Miia's official length) is estimated to weigh about 200 kilos (or 440 pounds), Miia and her mom must be even heavier due to having considerably thicker bodies compared to a real-life anaconda (in fact, hastily scribbled-out character notes from a volume cover imply that Miia is in the vicinity of 400-something kilograms, about twice as heavy as an anaconda!), and he dragged both of them. Course dragging bodies is much easier than lifting (he only had to lift their relatively light upper bodies).
In chapter 30, we learn he keeps returning from the Afterlife Antechamber under his own willpower in order to make the girls happy. That's taking Determinator to all-new heights. No wonder Lala fell for him.
Chapter 37: despite only being semi-conscious and half drowned, he still manages to save Mero using some of Rachnee's leftover string from chapter 36.
In Chapter 40, he has enough willpower to resist Lilith's hypnosis. Actually, he's so pure of heart that instead of getting hypnotized into having sex with her, he responds to hypnotic suggestion of forgoing his inhibitions by scolding her more firmly, thumping her on the head while telling her to knock it off already.
Also from Chapter 40, when Doppel gets captured by the Orc leader from MON's debut chapter, Kimihito saves the day by hurling an object at the Orc's head to distract him so Doppel can slip free. Considering what what a savage brute this Orc is, that takes serious nerves of steel.
Oh, and it's not just awesome...it's Crazy Awesome: The object Kimihito threw? It was Lilith, still blindfolded, bound, and on cloud nine from Doppel putting her in her place! And THEN Kimihito had the sense to pull Lilith out of harm's way before the Orc could grab her!
Suu saves a little girl from a car by absorbing her and then taking the impact herself.
Mero showing what's she's REALLY capable of in the water in chapter 17. Miia gets one as well for being able to keep up with her during the race (at least until their respective weaknesses kicked in; the cold pool water threatened to shut down Miia's cold-blooded body, and Mero started to choke and vomit after breathing too much chlorinated pool water).
Chapter 20 has the Kaiju Girl Fight. It must be seen to be believed. The anime manages to make the scene even more awesome by adding a Shout-Out to Godzilla, adding Godzilla's dorsal spikes to Suu and even giving her Godzilla's atomic breath.
Rachnera sexually dominating a devil. A being of pure lust, that lives to make others do her bidding, reduced to a gibbering slave by a spider-girl without any hypnotic or magical powers.
You may not agree with Ren's motives but she was a badass when facing Miia's cooking and Suu's... "cuddling".
Chapter 29's got one from Centorea's mother's unnamed 'Teaser'. Having fallen in love with her and faced with the prospect of a centaur stallion mating with Centorea's mom, he grabs a stick and manages to knock the stallion clean out. And stallions are hulking bruisers among extraspecies already known for their strength. That's Kimihito levels of 'don't hurt the one I care about'.
Rachnera gets into a duel with Octo in Chapter 36, with each using their own skills to tie up the other. Rachnera with her strategically placed string, and Octo with her articulate tentacles. Some of their moves reach Naruto levels of fancy, such as this. Even though it came from misunderstandingand lies, it is awesome nonetheless.
Mero's Heroic Sacrifice to save Kimihito from drowning in her mother's castle in Chapter 37. She gives the air in her swim bladder to give him one last breath, but doing so causes her to lose buoyancy, and since her tail fin had been damaged by falling rocks, she could neither swim nor float, leaving her unable to fight the current pulling her towards a deadly set of spikes. But that's okay, as long as her beloved is safe...Particularly poignant as Kimihito has given his life to save the girls several times, but this is the first time one of them returns the favor.
Chapter 40 has Doppel reveal her true nature to the orc leader from chapter 11 after he assaults Draco.
Manako utterly dominating the traditional festival games.
Centorea taking on an entire swarm of giant hornets with a sword (the same as those who stung 42 people to death in China). For context, killing a housefly with a knife is nearly impossible, let alone with something as heavy and cumbersome as a claymore or against targets as fast and aggressive as hornets, but that doesn't stop her from taking on hundreds of the damn things to protect her master. Proof that all her bluster about nobility and courage is far more than hyperbole.
The Cultural Exchange Expo is a truly awesome thing to witness, featuring dozens of booths, hundreds of monsters, and a huge diversity of products on sale catering to most every need or want a liminal could have. Not to mention the pair-up rooms that basically laugh in the face of the legal obstacles to human/liminal intimacy that were prevalent early in the manga.
In Chapter 51, Kyurii manages to resist her father's possession by biting herself instead of Papi.
Also for Chapter 51, Lala actually performing her job as a God of Death and banishing Kyurii's father to the afterlife with absolutely no effort.
So far, all but the second and sixth volumes (which were only beaten out by Naruto) have opened at #1 and volume six managed to reach the top of the best seller list on its second week anyway.
Even the first volume of the I ♥ Monster Girls 4-koma anthology managed to open at #1.
Another meta moment: the Monster Musume panel at Anime Expo 2016 not only featured Okayado himself, in his first trip overseas, but also an incredible Rachnera cosplay made by glamour model and professional cosplayer Marie-Claude Bourbonnais. Her page on the series' wiki shows pictures of her building Rachnera's arachnoid parts, sewing the costume and making everything mostly by herself in a relatively short time. The hard work she put in the project and the final result are nothing short of amazing.
While all the character songs sung by the voice actresses are pretty good, Saori Onishi takes hers to a whole new level. Onishi voices Doppel, and, since her knack is to turn into other characters, she makes impressions of all the other main harem members (and Ms. Smith) throughout the whole song. It's hard to believe that all those voices come out of a single person!