In the New 52, when Superman is first captured and studied by the military, they think he's a shapeshifter whose true form looks like a 5-legged goat creature. A flashback in a later episode reveals that the "alien" corpse they obtained was a malformed stillborn goat the Kents had passed off as an alien corpse after discovering baby Kal-El, bribing the soldiers investigating the crash with the body so they could leave the area undisturbed with the normal-looking humanoid infant that was readily accepted as their son.
The Pax Galactica, an ancient group of space warriors, declare Superman their leader after he defeats their mortal foe, the Lexus. Superman's first order of business.
Go home. Find another quest.
The Pax Galactica accepts.
At the climax of the lengthy President Luthor story arc, Superman and Batman enlist the aid of a twelve-year old billionaire genius (who was using the "Toyman" name at the time) to build them a spaceship to destroy the kryptonite meteor racing toward Earth. Their expressions are priceless when they see that he has built it to look like a towering giant robot that looks like Superman on the right side and Batman on the left.
The classic "Tales of the Bizarro World" stories.
The frequent Bronze Age crossovers with Ambush Bug, who considered Superman his best friend, much to Superman's annoyance.
Mr. Mxyzptlk's debut story back in the Golden Age.
Superman/Wonder Woman: Superman vs Apollo. A Curb-Stomp Battle that will remembered for the ages.
In the first Christopher Reeve film, Superman is being interviewed by Lois Lane on her apartment balcony. Regarding his x-ray vision, she jokingly asks what color underwear she's wearing, only for him to reply that he can't tell because the planters she happens to be standing behind apparently have a high lead content. This was forgotten for a few moments, but after she walks away for a moment, he suddenly informs her that her underwear is pink. She immediately steps back behind the planters... but also shyly asks if he likes pink.
"Superman, thank god... I mean, GET HIM!"
Superman has just broken into Luthor's underground lair.
Lex Luthor: Otis, take the gentleman's cape.
[Otis approaches Superman, who glares at him]
Otis: I don't think he wants me to, Mr. Luthor.
Also Hackman's cavalier, ad-libbed "Come in, it's open" just before.
When Superman shows up during his opening night to save Air Force One when an engine blows out. The pilot's expression as he looks out the window, sees a guy in a cape holding up the wing, and tells the rest of the crew "Just fly. We got... something... I ain't saying what, just... trust me."
In fact, a lot of the reactions people have when Clark suits up for the first time:
Pimp: Say Jim, that is a BAD outfit! (Supes flies off) Whoa!
Desk Sergeant (finding a boat loaded with tied-up robbers on his doorstep with a caped hero flying into the night): First drink's on me, lemme go get my hat...
Superman attempting his classic changing into costume in a phone booth, only to find it's just a phone on a post. It can be pretty weird to realize the concept was being mocked as early as 1978.
This absolutely classic line:
Superman: Easy, miss - I've got you.
Lois: You - you've got me ... (looks down) WHO'S GOT YOU?!?!?
In the first episode of Lois and Clark, the montage of Clark trying on different costumes before settling on the iconic one.
An episode of the old George Reeves The Adventures of Superman is spent trying to prevent crooks from misusing a scientist's new supercomputer (an "electronic brain" in those days) to deduce Superman's secret identity. After they've been stopped, the scientist (played by Sterling Holloway) is alone with the machine and realizes he wants to know the answer himself, so he asks it. Its answer is a strip of ticker tape that says, "Wouldn't you like to know!"