Dumb and Dumber/Funny

"I'll bet you twenty bucks I can get you gambling before the end of the day!"
- Even better in that he doesn't realize he's already won.
Lloyd's revenge on Harry.
"His head fell off?!"
- Even funnier is Harry's follow-up, "Yeah, he was pretty old..."

Lloyd: That's it. I've had it with this dump! We got no food, we got no jobs, our pet's HEADS ARE FALLING OFF!!

When Lloyd accidentally goes the wrong way during the road trip:

"Huh. I expected the Rocky Mountains to be a little rockier than this."
"I was thinking the same thing. That John Denver's full of shit, man."

When Lloyd pulls up next to Harry on a Vespa, and explains that he sold the Shaggin' Wagon for it and gas money. Harry starts off sounding like he's berating Lloyd, saying "There are times I think you can't get any dumber! And then you do something like this..." Then he turns it on its head when he follows up with "...and totally redeem yourself!"
"It's okay! I'm a limo driver!"
"We landed on the moon!" Ad-libbed by Jim Carrey, to boot.
Harry and Lloyd replacing the ransom money with I.O.U.s. Watching how they spend it is funny enough, but then the Big Bad finally opens the briefcase at the end and nearly blows a gasket.

"Go ahead and add it up. Every cent's accounted for. Look, see this? That's a car. 275 thou. Might wanna hang onto that one."

When Harry explains that he survived getting shot because the FBI gave him a bulletproof vest.

Lloyd: ...what if he had shot you in the head?
(beat)
Harry: (to FBI) Yeah, what if he had shot me in the head?
FBI: That was a risk we were willing to take.

The look on Harry's face when Lloyd has his hand in his coat (ready to pull his gun).
Lloyd outwits Sea Bass at the diner by somehow getting him to put their lunch on his tab.

Harry: That was genius, Lloyd, sheer genius. I mean, where did you come up with a scam like that?
Lloyd: Saw it in a movie once.
Harry: That's incredible! So what happened, so the guy tricks some sucker into picking up his tab and gets away with it scot-free?
Lloyd: No! In the movie, they catch up to him a half-mile down the road and slit his throat. (giggles) It was a good one!
Harry: (suddenly driving a lot faster)

When Lloyd first suggests to Harry that they go to Aspen.

Lloyd: What are we doing here, Harry? We've gotta get out of this town!
Harry: Yeah, and go where? Where do you wanna go?
Lloyd: I'll tell you where: someplace warm. Where the beer flows like wine. Where beautiful women instinctively flock like the salmon of Capistrano. I'm talking about a little place called Aspen.
Harry: I don't know, Lloyd; the French are assholes.