Have you got one single riff that flattens you every single time you hear it?
I’ll go with “Whup, I’m fartin’ Monopoly cards!” from Overdrawn at the Memory Bank and “Every frame looks like someone’s last known photo!” from Manos: Hands of Fate. And I’ll add more as I remember them. 
I’ve said elsewhere around here that “Omar coming” from The Christmas That Almost Wasn’t was perhaps the biggest laugh I’ve ever gotten from the show, largely due to just how unexpected it was that they’d reference The Wire after it was on so long ago (coming just a couple years too late to be referenced by the original run), and it so perfectly fits the visual of the kid whistling and strutting around with a Christmas tree slung over his shoulder like Omar’s shotgun.
My father has two that destroy him every single time: “Yeah, watch this. HEY FRANK!” from Beginning of the End, and “He brought his lucky cinder block along” from The Mole People. Ever since the latter, my family has referred to heavy luggage as a lucky cinder block, until I’m not 100% sure my mother remembers what that’s from.
“Noooobody expects the Spanish Inquisition” from Santa Claus Conquers the Martians kills me ever dang time. The first time, I about peed myself because it was so unexpected.
My goodness, there simply cannot be a single favorite riff.
But one that makes me cry just thinking about it is from the short Using Your Voice (ep. 313) where the stentorian onscreen narrator starts with “Do you know . . . .” and Crow leaps in: “-- that I have little bunnies painted on my knees? I do.”
It’s the way he says “I do” – priceless.
In Design for Dreaming, the woman is dancing above a crowd that’s clapping rhythmically and the riff is “Give us Cyd Charisse!”
That’s always been the perfect riff to me - two clever references mashed together to make one really funny one. It epitomizes the concept of the show.
“I’m gonna need a bigger shoe…we’re gonna get a bigger shoe, right?”
“Sam, I’ve been blown to several thousand bits, but I gotta finish these patent papers!”
Guaranteed to bust my gut Every. Single. Time. 
Of course I’ll hedge this with qualifiers but if I had to list just one… today I’m gonna go with the moment in The Mole People when the professor gives a particularly insane theory about how the earth is structured and Mike says:
“This turns out to be the correct theory”
As someone who saw this episode as a kid long before I saw Jaws, and had no idea what the line was referencing, it was still hysterical to me.
Rewatching Future War: What kind of loser saint has this sanctuary… The Arby’s Employee Chapel
Oh, my… sorry couldn’t rewind to get exact quotes
“Mitchell”. Even his NAME says “Is that a beer?”
That, and “I seem to have died, is that okay?” from Overdrawn at the Memory Bank
“I wonder if there’s beer on the sun”
Had my wires crossed earlier and confused riffs from different movies … got to thinking about and realized my mistake. Take 2:
“Strut, pout, put it out, that’s what you want from grandma” from Racket Girls catches me off-guard and makes me laugh every time.
That, and “… although I do weather-vane in high winds” from Operation Kid Brother.
Amen to that, though that whole film is packed.
There’s the snarl that’s made whenever Troy looks like this:
And there’s during Satoris’s ritual when they riff “God bless Mommy and Daddy and Gran’pa and all my evil henchmen…”
And there’s Tom replying as Pipper, “Knew him? He was delicious!”
And there’s “the Canada Song”…
Has to be in Viking Women and the Sea Serpent – A pig is on screen and Joel says “Come on! Squeal like Ned Beatty.”
From Overdrawn at the Memory Bank:
“FINGAL, YOU ARE MINE!”
“…WITH BEARNAISE SAUCE.”
That’s actually what my husband and I call the movie because he loves that riff so much.