Agent Six finds Rex, Noah, and Bobo in Van Kleiss's place, and try to escape, the following conversation ensues.
Rex: How'd you find me? Agent Six: High Frequency Transponder. Rex: You tagged me? Like a dolphin? Agent Six: Not you, it's in the monkey's diaper. (Rex and Noah stop for a moment, and look at Bobo) Bobo: It's a simian undergarment!
"Operation: Wingman". All of it.
Key to "all of it" (aside from the Killer Rabbit) is Rex's "date" Anna, "the Blond Widow". She takes The JinxUp to Eleven. Case in point: Step One - she loses her lipstick. Step Two - she and Rex go back to the limo to search for it, Rex casing the back, her the front. Step Three - she finds her purse lodged in the center console, blocked by the parking brake, so she lowers it to get it out, but not only forgets to raise it again, but accidentally locks all the doors. Cue limousine rolling right off a cliff into the ocean.
Slightly more subtle: the anchor specifically says Providence used a smell sample from Rex's "colorful undies".
Young!Rex's conversation with a recently-captured Bobo:
Rex: I don't know where they keep the jet planes. Knight: Why did you let him out?! Rex: He asked me to. I don't if you guys are aware, but that monkey talks!
The ending of "The Plague", when the mystery of why Bobo, unlike every other Evo, was affected by the coma-inducing plague: he wasn't affected. He was sleeping off a hangover. For days.
At the end of "Waste Land" Serge the fishman and Six have finally put aside their differences after stopping the reactor from exploding. Rex is running around, taking photos and being hyper and dispensing Aesops.
Serge: I do have more Paralyzing Urchin spines if you want them.
In one picture Rex lobs a starfish at Six's face, which sticks! Six lunging after Rex in the next photo is the closest we've seen to him losing his composure and it is hilarious.
In "Forgotten", the hilariously victorious trumpet version of one of the main themes usually playing during Rex's Badass moments after Holiday promises to go on a date with him.
"Robo Bobo". The entire episode played out like a slapstick comedy, complete with the three segment montage featuring Holiday and Six chasing baby bunnies, Rex fighting mama bunny and Bobo fighting his evil clone.
In "Mixed Signals", the revelation that before the Nanite Incident, Rex's primary goal in life was to become an accordion player. Complete with the show's theme being played on accordion in the background.
Not to mention Rex's WTF face when this happens.
Also in "Mixed Signals", Rex's brother. He, in the middle of his ship being torn at by EVO's, asks "Hey, you wanna see a picture of you, me, and Papi?" His tone got this troper on the floor laughing. All in all, you can find just about everything he does kind of funny.
"Without a Paddle". All of it. It might even exceed "Operation Wingman" in hilarity.
Mr. Buchiner got his PONY.
Pretty much the whole of "Haunted", especially Rex and Noah's ridiculous scheme and the way the girls kept one-upping them. And at the end, when Noah finally gets an Almost Kiss with Claire, she and Annie ditch him and Rex to fangirl over Cesar. Noah's expression as he says "still waiting" (his catchphrase of the episode) is priceless.
In "Night Falls", Rex has a cute nickname which everyone knows about but he doesn't understand It refers to him wearing his pants over his head and pretending he's a luchador. And Cesar zaps him with a taser. Twice.
In "Family Holiday" when Bobo references Rex's comment about Holiday fangirling about Brandon Moses to Cesar.
Bobo: (after Cesar runs off after Brandon Moses) Now that's acting fangirl.
In "Rampage," Rex ends up giving his cell phone to a persistent old lady who wants Providence to help her with some obscene EVO problem, telling her to call the first number on speed dial. At the end of the episode, we see Six getting called by her, glaring at a grinning Rex.
Also, Rex watching a telenovela. He does it again in "Moonlighting."
Most of "Exposed" barring the mid-episode Mood Whiplash, but the kicker goes to the heavily edited news report at the end which depicts Six as a suave Badass with a sensitive side that throws out one liners like they had a sale on them at the Providence gift shop. The subject's response: "No comment," but it's even funnier if you think that Six secretly loved it.
"Six Minus Six", the entire episode.
Six: Maybe Doctor Beautiful was right, I do need my head examined.
"Crash and Burn", when Six suggests Rex use his sword while on his bike. Red says he's never done two builds at once before. Six asks why. Rex doesn't know.
Also during the race. Before the start, Bobo offered his cheese covered...somethings to Six who refused. Later...
Bobo: You gonna help him?
Six: He's doing fine.
Bobo: Are you watching the same race I am?
*Rex starts using his Slam Cannon while on the Rex Ride and pulls ahead*
Six: Like I said, he's doing fine. *steals some of Bobo's food*
The scene from "Heroes United" in which Ben smacks himself on the ass to get Alpha's attention. Seriously died laughing at that point.
"Phantom of the Soap Opera" in which Rex meets the actors from his favorite Telenovela, who are the embodiment of Ham and Cheese both on- and off-camera, and has to save the studio from an extremely clever EVO mongoose.
Doctor Holiday: Always go for the big red button.
That goofy ass run Noah does in the beginning of "Guy vs. Guy".
In "Black and White", Holiday has to leave Six "partially naked"... by taking his swords to disable an electrical defense system. Later, when Six needs to cut off a wall panel, he reflexively tries to draw his swords:
Not only the fact that Gatlocke holds minion auctions at a high school—not only that he's looking over a nerdy student's resume by the time Rex finds him—but the fact that Gatlocke gives the kid his first assignment—stop Rex—and he actually tries! Rex ignores him completely.
During "End Game", Dr. Gabriel Rylander repeatedly makes jokes about his current condition.
Meechum: (observing the results of Rex's Worldwide Cure on the monitor) "Are you seeing this?!"
Rylander: "Technically I don't have eyes, but yes."