Ciel's inward horrified reactions at Viscount Druitt's advances in episode 4, as well as his reaction when he finds out Lizzy is there too.
Sebastian training Pluto with a Carrot/Stick method.
From the manga's circus arc, every instance of Ciel trying (and failing) to fit in with the circus troupe, from having to dress himself to having to peel potatoes for what is very likely the first time in his life.
Freckles:"Smile, what're you planning to make from them potatoes with peels thicker than the meat ?"
From the same chapter, Sebastian and William find out they'll have to share a room.
While Ciel and Sebastian are at the circus, they get a call from the manor. Elizabeth is visiting and the staff want them to come home. In the background, we see her wreaking havoc.
Sebastian and the tiger.
In episode 3 of Kuroshitsuji II, Ciel turns to talk to Sebastian...only to find him fawning over a group of cats with an awestruck expression on his face. Considering how cold and composed he normally is, the moment is absolutely priceless.
And Ciel does a Face Palm and says that Sebastian is a "cat obsessed moron".
During one episode of the first season, Sebastian explains why he has such a preference for cats over dogs to Ciel. After he finishes talking, Ciel, in a rare moment of humor, barks at him.
In vol. 2 of the manga, Sebastian goes through his usual routine. All the other house staff come to him when something has gone wrong, and his inner thoughts on them are pretty humorous.
Sebastian (on Bardroy): The remaining 20% of your "food" has been hazardous waste.
Sebastian (on Finny): I have seen many things in my long life, but this is the first time I've dealt with a creature from outer space.
When Ciel is detailing the characteristics of a string of murder victims, Sebastian voices his affirmative in increasingly loud yeses. Ciel looks around to find his butler... fondling a cat with "black, midnight hair" and etc.
Stoic and composed William T. Spears getting his glasses knocked off, the Velma style way he looks for them, and yelling at the brick wall he thinks is Ronald from episode 9 of the second season.
Season 1 episode 14:
Ciel: "Have you concocted a curry better than Agni's yet?" (flashback shot of chibi Soma in a food coma) Sebastian: "No. According to Prince Soma, something is still lacking." Ciel: "Then get back to the kitchen and stop wasting your time making cakes. You have more important things to do." Sebastian: "Yes, I understand, my Lord." Ciel: "And anyway, I'm not really in the mood for chocolate right now. Drop it into the curry or something." Sebastian: "Indeed, my young master."
Even better is Ciel's shocked "oh-God-do-we-really-have-to-do-that?!" expression upon being told about it.
Later, the group has to do it again, and this time, Snake is part of the group. It's quite hilarious for not only Snake to strike the "Phoenix pose", but also the snakes on his shoulders striking the pose as well! (or as well as they can, since they have no arms).
In chapter 37 Nina Hopkins states that she only likes boys and girls that are 15 years or younger, prompting Sebastian to comment that he does not need to know what she does in her spare time.
Chapter 62. The poor owl.
In one chapter Sebastian sits up in his coffin after faking his death, which results in Bard, Finny, Maylene, Lizzie, Agni, and the Prince tackle-glomping him. All while still in the coffin.
Claude kisses Ciel's foot in Episode 10 of Season 2...and gets kicked in the face for it.
While he was supposed to be faking his death, Sebastian risks exposing the whole scheme in order to... frantically hide a dozen cats in his wardrobe.
Which was later opened by the guests searching for clues to Sebastian's "murder", flinging the cats everywhere!
At the end of "Solo Butler" (season 2, episode 2), Sebastian gives Ciel the following advice for handling Grell:
If your eyes happen to meet, don't break eye contact; just back away slowly and call out from a safe distance, "Say, have you gained weight?"
Depending on your interpretation, the ending of season 2 is downright hilarious. Sebastian has to serve Ciel for all eternity, and will never have the chance to claim his master's soul? Classic. As this troper once heard someone say, "the ending of season 2 is basically Sebastian being tied to a chair and having a delicious cake set in front of him. And he never gets to eat it."
Alois dancing around in the first episode and saying "Ole!"
Sebastian explaining that in his dimension, the only pets they have are xenomorphs.
Chapter 72. Violet, the purple house's prefect, demands the green house's prefect's fag to pose awkwardly for him while Violet draws. A case of Exact Words: "You pose, I draw whatever the hell I want."
The look on Ciel's face when Lizzy starts chopping up zombies with a pair of swords. Just, the lookon his face.
I think the kicker of the whole thing was a rarely serious Elizabeth offering to carry an injured Ciel on her back while Sebastian calmly tells her that he could do it.
Aboard the cruise ship, Ciel and Sebastian head off for the Aurora Society meeting. As they do, Ciel leaves his cake with Snake. After a moment, Snake noms it. The expressions sell it.
Sebastian dressing as an ugly unicorn to sell perfume in chapter 85.
Also, when Sebastian learns that Wolfram is taking so damn long to prepare dinner because he makes each dish separately. He's dumbfounded that someone managed to take inefficiency to such a level.
The Making Of Kuroshitsuji II is itself one giant CMOF, what with Animated Actors (who nevertheless mirror their "characters'" selves uncannily) but the best bit is the fake trailer at the end, which at first seems feasible, but quickly descends into Shocking Swerve after Shocking Swerve before it degenerates into a massive Cliché Storm of Anime Tropes, even ones belonging in entirely different genres. The animators must've had the time of their lives...
The flashback to Sebastian's first days as Ciel's butler in the manga is full of great little moments.
After freeing Ciel from the cultists' hideout, Sebastian says they should go back to his mansion.
Sebastian: *still smiling*I take that back. I ended up serving an incredibly disagreeable boy.
Sebastian brings Ciel into the restored Phantomhive mansion. One panel later, Ciel is screaming loudly enough to scare owls outside beside Sebastian just poured a bunch of boiling water into his bath. Then he brings Ciel some warm milk and honey to help him get to sleep, and Ciel stops him before he leaves the room...only to tell him that he better learn to prepare human food properly soon, because he can't live on warm milk for the rest of his life.
Sebastian: *closes door behind him* That little brat...
Ciel makes a mistake in his studies, so Sebastian smacks him across the palms of his hands. Later, Sebastian - who is still struggling to get the hang of cooking - screws up making tea, so Ciel dumps the tea into his hands. Sebastian's expression is priceless◊.