Snake: The cornered rat will bite the cat...sounds like trouble. Mei Ling: Hey! When did you become so good at proverbs? Snake: You must have rubbed off on me. Mei Ling: Well, come back in one piece, and I'll teach you all the Chinese proverbs you can handle.
Oh, God, Luigi's:
Snake: That guy with the mustache... Colonel: Ha. You mean the king of second bananas! Snake: Hey! That's Luigi! Show some respect! Colonel: Look at that pale skin. He's been living in his brother's shadow for too long. Snake: That's a low blow, Colonel! Colonel: Face it, Snake! Once a kid brother, always a kid brother! Snake: Colonel, what's gotten into you?! Colonel: La li lu le lo! La li lu le lo! La li lu le lo! Snake: Colonel, snap out of it! Colonel?! COOOOOOOOOOLONEEEEEEEEL!
On Marth, these two lines:
Mei Ling: Snake, have you heard the saying "politics make strange bedfellows"? Snake: Don't tell me that's a Chinese proverb.
Snake: What kind of weirdo puts their face on the bow of a ship, anyway? Mei Ling: Maybe he's a little narcissistic. Kind of like someone else I know... Snake: Really... And who would that be? Mei Ling: Use your imagination...
Mei Ling: Snake, have you ever heard the ancient Chinese story, "The Vain Ocean Of Wealth"? Snake: I've been waiting for the movie.
Discussing Peach and her relationship to Mario:
Mei Ling: But still, don't you think it's romantic? I mean, to have a guy who's always there for you? Snake: If he was smart, he'd tell her to stop getting kidnapped. Mei Ling: ...You don't get a lot of dates, do you, Snake?
Mei Ling: Could you catch a Pikachu for me? Pleeeeeease? Snake: Oh, come on! What do I look like, a Pokémon Trainer? Mei Ling: Fine... Sorry I asked.
Otacon: He used to be a weakling, but countless trials over the years have toughened him up. Snake: By trials, you mean game overs? Otacon: Yeah, you'd know all about that.
On Pokémon Trainer:
Colonel: Let's do this together, partner. Snake: Whatever you say, Colonel.
Even better in context. (Colonel has just delivered a spiel on how soldiers and commanders rely on each other for maximum effectiveness, effectively making them a team. Snake is... skeptical of this viewpoint.)
Snake: Aw, there's a big eyeball walking around here... Colonel: That's just Jigglypuff. Snake: Jigglypuff? That some kind of marshmallow?
Oh, lordy, Sonic:
Snake: *grumbles* Otacon: Snake, what is it? Snake: Something about that hedgehog rubs me the wrong way. Otacon: ...oh, you mean Sonic the Hedgehog! But everyone loves Sonic! He's a big star! Do you have any idea how excited people are that he's here in Brawl? Snake: Yeah, I know, but there's something about him I just don't like. Otacon: But...why? You must have some kind of reason? Snake: ...nope. Just don't like him.
The Wild Mass Guessing this has spawned is incredible. So far, the most prominent theory seems to be that this is an Actor Allusion; Snake's Japanese voice actor's father voices Dr. Eggman.
One simple line on Wolf:
Snake: Wolf. Real imaginative name.
And finally, Zero Suit Samus:
Snake: Mei Ling, Samus took her clothes off! Mei Ling: That's Zero Suit Samus, Snake. Snake: Without that bulky Power Suit, she's gotten a lot more agile... maybe if I took off all this heavy gear I could chase her. Mei Ling: Uh, yeah, you wish. Snake: Her loss.
This entire cutscene from the Subspace Emissary also deserves mentioning. Luigi further solidifying his Butt Monkey status by being scared... of a WaddleDee (the remix of the Luigi's Mansion theme helps too), and then Wario getting his ride jacked by King Dedede had this troper in stitches.
Luigi is there simply for comedic relief. His Subspace Trophy pose is different from his actual trophy pose in the game, to a funny extent. He was thrown into the cart upside down. Dedede even stuck one of his badges onto Luigi's trophified nose.
Ganondorf turning the statue gun on Bowser. Even funnier is that about two seconds later, he gets swatted by Tabuu.
Even funnier is what Bowser does to Ganondorf's trophy afterwards. Disrespect does not begin to cover it. It's basically a montage of him attacking it in so many different ways...
Ganondorf is revived in a later cutscene, and is clearly a little groggy.
When Kirby and Dedede's group meet up near the end, Dedede immediately glomps Kirby, and then drags him off while Kirby has a complete "WTF?" look on his face. Priceless.
Whenever Peach hits someone in the head with her frying pan. Bonus points if it's Bowser, Mario, or someone more serious like Meta Knight. Just imagine the characters' reaction if they spoke.
"One More Brawl Taunts" covered that for Bowser.
Peach: Grrr... *pwns Bowser with the frying pan* Bowser: WHERE DID YOU GET THAT FRYING PAN?! Peach: Umm...in my kitchen?
Brawl- takes the liberty of putting voice clips in hilarious places. Example: Snake's "Nom nom nom... Tasty." and Captain Falcon's "YESZ!!!" But by far Peach's forward throw takes the cake. "This is fun! *slap*"
During the Subspace Emissary, Samus and Pikachu enter a room. It seems pretty safe and they're just walking along calmly when *SWOOP* ... Uh, what the heck just happened? Where's Samus? The pure Mood Whiplash of Ridley's entrance got a chuckle out of me when I first saw it.
Wario gets a few moments of funny in the game:
His jaw dropping upon seeing Luigi and Ness help Dedede up.
His complete indifference to Master Hand's corpse.
Word of God stating that even though Wario joined the Subspace Army, he "cares very little for the actual objective he was given and wants to continue just doing whatever he likes."
After defeating Bowser, King Dedede revives his statue, leading him to retake his intimidating roars from before. When Dedede then nonchalantly dope slaps him in response, the giant Koopa makes a sulky "what the hell was that for?" gesture.
Right before Meta Knight battles Lucario, the cutscene cuts back to the Ice Climbers, who do a Loud Gulp.
The original E3 2006 trailer opens with Mario, Kirby, Link, and Pikachu in their Melee models running towards each other on a black screen. Then, the Battlefield from Brawl appears in the background as everyone changes to their updated Brawl models... Except for Kirby, who didn't have a game of his own on the Wii at the time. Kirby himself looks confused.
Wario's role in the trailer. After several awesome moments, he crashes in on his bike, surrounded by the other characters. Wario gets up, sees the others, grins sheepishly... and then the waft begins charging. The Smashers recoil a bit and it cuts to Wario about to unleash it. We then get a split-second shot of everyone's Mass "Oh, Crap!" before the explosion.
Luigi's introduction in Melee's Adventure Mode. The cutscene starts as normal, with Mario jumping into the air, ready to battle the player...only to be knocked backwards by Luigi stepping on his head in mid-jump.
In Melee, a sufficiently-weakened Mewtwo changes his method of climbing up ledges. He walks up. Even as a casual display of his power, it's an odd move for someone as no-nonsense as he usually is.
In regular fights, when a character gets knocked off-screen upwards, in rare moments, instead of getting Star-KO'd, they end up getting launched into the screen.
It's hilarious to watch characters flail around with Superspicy Curry. Particularly if it's someone like Ganondorf, Marth, or even ROB.
The fact that there are two extremely tiny CPU-controlled characters in the Kaiju-based event match Gargatuans in Melee. While they are obviously just there to represent civilians who are caught within the crossfire of the epic battle, it is possible to be able to win the match without even doing anything because of DK trying to get these two characters.
In Brawl, Assist Trophies have a chance to spawn Mr. Resetti. Normally, this means you spend more than a minute with a huge text box spouting an angry rant at you. However, since Resetti is a mole, he's stuck on whatever platform he spawns on, which means his platform can collapse or scroll offscreen. If he does, not only does his rant end early, but there's a special, highly amusing line of dialogue for when this happens.
"H-hey! Don't you ditch me, punk!"
Mario's face when he almost gets hit by flying Metal Blades in Megaman's Reveal Trailer for Smash Bros. 4.
For that matter, the fact that the Villager manages to give Mario a death glare◊ through that blissful expression.
Not to mention that Villager gained Memetic Mutation status as a cold-blooded, psychotic killer within MINUTES of the announcement.
In the Wii Fit Trainer's Reveal Trailer, Mario, Link, and Kirby are "playing" Wii Fit, trying to hold the tree pose when she casually walks up, taps Mario on the shoulder, then strikes another pose, OHKOing all three of them.
What makes it even funnier is that after this, the game says "Great, keep it up!". It's as if she was encouraged for beating the crap out of people.
What also makes it funnier is how they do the pose. Mario can't get his arms over his hat, Kirby (having no legs) just puts his foot over the other and can't get his arms over his head because he's too round. Link (having a more realistically human shape than the others) is the only one doing the pose right, but still having trouble keeping it.
On a related note, this official poster◊ shows silhouettes of the aforementioned characters doing the tree pose, along with Pit, Donkey Kong, and Samus.
Hell, the Wii Fit Trainer as a whole because her moves are ridiculously funny: yoga poses for every single thing.