Rath and his debut episode, hands down. He's like a hilarious send-up of a professional wrestler that addresses everyone by their full name while calling them out. Highlights include trying to remove the Omnitrix symbol and calling Vulcanus " Baby Man."
Rath: * Gwen creates a force field around Rath's head* "Did I just use up all my air?" * falls unconscious*
Rath starting to warm up to Tiffin, only to grab him and hide him behind his back when Gwen and Kevin look back.
Gwen mentions Argit would "sell his mother for lunch money." The result?
Kevin: I happen to know he got top dollar for his mom.
The trio's response to Tiffin being eaten. The complete silence and "D:" faces were hilarious enough, but then Kevin faints.
Rath's reaction as well; he jumps down the Pantophage ruler's throat to save the Tiffin, then threatens to do it again and knit his intestines into a sweater if he doesn't agree to a peace treaty with the Luodans. May double as a Crowning Moment Of Awesome. And his line just before that?
"Dude. Eating. Babies. Is NOT. COOL!"
Ben and Kevin's reaction to seeing the horridly-aged Morningstar is an uncharacteristically girly "Eeww!"
Ben trying to disuade a rapidly aged Kevin from driving:
In "Fool's Gold", when the team races for the little alien who grew giant and primal after eating meat, they are warned that meat makes them poop Uranium-1417 (to quote Kevin, "Unstable radioactive poop?!"). No sooner than they go after it, Kevin inadvertently steps on the radioactive dung and bounces through the corn field by way of always landing on the explosive crap.
The ending of "Good Copy, Bad Copy": "BRING ME CHILLI FRIES!"
From "Save the Last Dance": Kevin attempting to a tie a bow-tie, FAILING, and Ben fixing it: "Well, don't you look dapper?"
In the episode "Undercover": Ben riding piggy-back on Kevin. Also, the plan using Kevin made of alien metal that emanates alien-defeating noise when it vibrates (thanks to Gwen hitting Kevin, really hard) and Ben as Echo Echo amplifying the sound. Something about this strategy is insanely funny. It must be Kevin losing his ability to walk as Gwen keeps hitting him. Made even better by Cooper's fanboying over Gwen and comparing her to a princess that saved him from the Cave of Despair in an MMORPG.
From "Pet Project": According to Kevin, Ship = "Cup of Instant Weapon" and "everyone shoots at us."
In "Simple," while Ben and Gwen try to stop an alien war, Kevin spends the whole episode profiteering. At the end, when everything goes south, he's forced to drop his ill gotten gains in order to get away. He... doesn't take it well.
Gwen (thinking he's reacting to everything going badly): "You're crying? Aww... you do have a heart."
Kevin (through sobs): "Yeah, that's what poor people have instead of money."
From "Grounded": "You can't ground me! I'm a super-powered alien!"
From "Vendetta", Ben (as Rath) is fighting a group of Forever Knights. The "Run away!" bit is funny enough. They meet Kevin, and make for another aisle, there to meet Gwen:
Gwen: Lady or the Tiger, boys. Your choice. Okay, not really. * BLAST*
Paradox: An excellent question, though hardly the point.
During Ben and Vilgax's fight in Primus, Ben as Way Big uses an old schoolyard trick against Vilgax. And not just any schoolyard trick...
Way Big grabs Vilgax's fist and slams it into his head repeatedly. Way Big: Stop hitting yourself! Stop hitting yourself! Stop hitting yourself!
The judge's resolution over who gets custody of Ship in Vreedle, Vreedle. Even after Humongosaur breaks his work bench, he still decides the case in his favor, just because he shouldn't tire himself over these trivial needs.
The Highbreed baliff's freakout over the insult of his species.
Baz-El: And while the Highbreed may be foul, smelly, evil overlords...