Spaceballs

Holy shit, you want me to pick just one line from the best parody movie of all time?!

I guess if I were forced to pick, it would be:
"I am your father’s brother’s nephew’s cousin’s former roommate."
"So what does that make us?"
"Absolutely nothing."

I'm surrounded by assholes!

Honorable mention because I use this one surprisingly often:

1-2-3-4-5? That's amazing. I've got the same combination on my luggage.

No more beaming! This time, I’ll walk! opens the door and it’s right there

Comb the desert. "We ain't found shit!"

Love the combing the desert scene.

https://www.youtube.com/watch?v=hD5eqBDPMDg

Oh, man, sooo many

"Whats the matter, Colonel Sanders? Chicken???"

"Just what I needed. A Druish princess."
"Funny, she doesn't look Druish."

"Alright, the short, short version. Do you? Do You? Good! You're married, now kiss her"

And the entire "I'm surrounded by assholes" scene

So many of the others I first thought were already here so… I see your Schwartz is as big as mine

Raspberry. There's only one man who would dare give me the raspberry! Lone Starr!

gestures broadly at entire script

I agree with @ringwraithfish about that being a very hard choice.

...

Nope, can't decide between (all paraphrased because I mainly remember the German translation)

• "Comb through the desert!" [cut, followed by troopers using a giant comb on the sand]
• The passcode scene
• "Water, water!" "Water, water!" "Oil, oil!" "Roomservice, roomservice!"
• "Didn't I say only pack what we need for survival?" "But I can't live without this thing!"
• "Yoghurt! I hate Yoghurt!" [followed by underling offering to change Big Helmet's breakfast and a rebuke that that wasn't what he meant]
• "He's an Asshole, sir." "I know that, I want to know his surname!" "That is his surname, sir." [Big Helmet asks who else on the ship is an Asshole and nearly everyone holds up their hand.] "I knew I was surrounded by assholes!"

Goodness..

They've gone to plaid!

...and change the combination on my luggage

Oh no....not again!

Dink dink... Dink dink dink dink dink dink... Dink dink.

Sir. The radar, sir. It appears to be...jammed.

MAIWWAGGGEEE

Lonestar! I am your father's brother's nephew's cousin's former roommate.

There’s so many great ones, but one I use daily is “you went over my helmet?!”

“Why didn't somebody tell me my ass was so big?”

One of my friends recently got scammed online. He should have known better. But when he got the ticket and the number on it was 123 456 789 10 11 12, I had to tell him that was the same as the combination on my luggage

This will seem strange, because it isn't one of the quotable lines you usually think of.

But, "That's gonna leave a mark" when Barf forgets the seatbelt. That bit is so classically John Candy. It may not seem like it, but watch the scene and see how he plays it. Subtle, which is so rare with a bit physical humor.

I laugh as hard at the way he delivers that otherwise not very funny line as I do almost anything else in the movie.

It doesn't hurt that the line itself has become a family in-joke of sorts. We've got kids in the family that use the line when they bump into something or whatever, that haven't seen the movie yet.

Now, the scene that makes me laugh the most is the "oh no, not again" bit in the diner. The whole scene is great, what with that reference to alien, the dancing baby xenomorph, and the Pullman/Candy reaction.

But the seatbelt line encapsulates what I've always loved about Candy.

Such a great movie. To add to the list...

It's Mega Maid, sir. She's gone from suck to blow.

Light speed is too slow. We'll have to go right to Ludacris speed.
Ludacris speed?!?
What's the matter Colonel Sanders? Chicken?

"I'm not a human, I'm a mog!

Half human, half dog.

I'm my own best friend."

When will then become now!?

The Space Balls FLAMETHROWER!!!

We have gone plaid.